TLDR: Things are about to get a lot better for Tanuma and Natsume! But they're about to get a lot worse first.
Natsume: Traumatized, inexperienced, reactive. Changes behavior only after confrontation or crisis.
Tanuma: Proactive but repressed. Accepts apparent limits but jumps on opportunities to surpass them.
Volume 1, Natsume, Human Exterminator First Meeting
Volume 5 Same Scenery Introduction of Problem
Volume 8 Culture Festival
Something Reflected Confrontation/Crisis 1
Volume 9 Eastern Forest Resistance
Volume 11 Something Sealed
Volume 12 The Other Side of the Glass Major Confrontation/Crisis
Volume 14 Unchanging Form Attempt to Correct
Volume 16 Distant Festival Lights
Volume 22 The Face of the Noren
Volume 24 Suspicious Visitor Confrontation/Crisis 3
Volume 25 Village of Sleeping Vessels Attempt to Correct
Cookies to the Forest Entrance
Volume 27 Master of the Name Calls
Dream of a Rainbow Cloudy Night
Volume 29 By Invitation of the Queen
Volume 30 Portrait of a Girl Point of No Return
Volume 33 Special 23 False High
Volume 34? Final Confrontation/Crisis <-- We Are Here
To elaborate on these "plot points":
First Meeting: Self-explanatory
Introduction of Problem: Natsume is hiding things from Tanuma, Tanuma accepts his boundary because Natsume is dealing with a lot and Tanuma can't do much anyway.
Confrontation/Crisis 1: Tanuma's mirror yokai tells Natsume that it hurts to not understand what's happening. Natsume keeps Tanuma at a distance for his protection but Tanuma points out that Natsume can also get hurt.
Resistance: During this phase Natsume is really struggling to open up even though he knows it bothers Tanuma (and others) and has to be told to do anything but the bare minimum. Nothing has really changed since Crisis 1.
Major Confrontation/Crisis: Tanuma breaks down and Natsume considers giving up (ties directly into main arc).
Attempt to Correct: Major crisis --> Major gesture = talking about Reiko. Reiko is a very vulnerable issue for Natsume, so it's a significant risk for Natsume to talk to Tanuma like this.
Throughout this period we see Natsume make an active attempt to compromise and include Tanuma without putting him in danger. He also goes through a phase of trying to avoid helping yokai so others don't get caught up in his problems.
Confrontation/Crisis 3: Natsume scolds Tanuma for not asking for help. Tanuma ignores problems for the sake of being involved in Natsume's world. Natsume worries about Tanuma feeling alienated by all his yokai talk but neither actually addresses the issue. At the end Natsume concludes Tanuma shouldn't be directly involved with yokai.
Attempt to Correct: Tanuma tries to open up more, Natsume tries to include Tanuma more even when he's scared. Still "fumbling."
Point of No Return: This is not the "Point of No Return" in a lot of plot charts that happens near the beginning of the story, but there's really no better term. Tanuma cannot fully return to the ordinary world. Our two biggest current antagonists know (a) his true identity, (b) his connection to Natsume, and (c) to an ambiguous extent, his gift.
False High: We're shinyuu/close friends now! Tanuma is opening up about his family! Pay no attention to the fact that Natsume is still paranoid and overprotective and Tanuma is pretending things are okay when they aren't. Parallel with Tanuma's mother helps reinforce dilemma that triggers crisis: Tanuma would rather risk everything than be left behind, but holds back out of respect for Natsume.
Final Confrontation/Crisis: Something bad happens that forces our characters to acknowledge all the flaws in their relationship. May lead to temporary separation.
Transformation: In a bittersweet/tragic story this is where they give up for good. In a happy ending, this is where they suck it up and do the brave thing because they've decided it's worth it.
Climax: Characters use transformation to save (or lose) the day. Note: may be integrated with main arc climax or may precede it.
Denouement: We get to see the new, stable relationship in action! Yay! (Or we see the characters move on. Boo.) If relationship arc precedes main arc, this would generally be where we go through the final act of the main arc.
In a typical story arc the first major crisis (Omibashira) would be about halfway, but since this a continuous work with no preset end point it mostly just follows pacing guidelines of alternating between major and minor crises and increasing tension. Technically we could have more than two major crises, so why do I think we're at the final crisis for Tanuma? I was working on a longer version with quotes and so on, but for the sake of readability this is what we've been shown and sometimes outright told:
Tanuma worries about Natsume and desperately wants to support him with yokai (as well as share some of the more positive experiences), but believes that because he does not have the same power he would only hold Natsume back. Tanuma represses his desires because he sees them as "greedy," and rarely communicates them to Natsume. Tanuma seems to believe that Natsume will inevitably move on because of how different their worlds are. Tanuma chooses not to convey his stronger feelings, so Natsume is not aware of how deeply Tanuma feels, and frequently misses the subtext of his actions.
Natsume wants to keep everyone around him safe, and particularly worries about Tanuma because of his vulnerability to yokai energy. He believes the only way to do this is to keep the yokai world and ordinary world separate, which means keeping Tanuma compartmentalized. Though he tries to open up and accommodate Tanuma's desires, he's still cagey about the exorcist world and doesn't give Tanuma many opportunities to be useful. Natsume is paranoid about Tanuma's wellbeing in general, and tries to hide how much he worries. Natsume is not very good at communicating, and frequently misses opportunities to clarify their situation. Distant Festival Lights implies Natsume is supposed to protect Tanuma by being at his side, but instead he keeps Tanuma at a distance, safely bottled up.
Both are worried about each other, but neither wants to burden the other with things that seem unsolvable.
Without getting into specific spoilers, the current arc has set a trap for Tanuma: an opportunity to do something meaningful for Natsume, something that only he can do, but at the cost of doing something questionable. It is currently ambiguous exactly how far he decided to go, but he has at least partly fallen into the trap—enough that he will likely take the blame (fairly or otherwise) when something inevitably goes wrong. It is wildly out of character for Tanuma, but like Omibashira Tanuma's pettiness, it is also completely explained by his unmet emotional needs. This means the only way Tanuma can explain what he's done is to tell Natsume the full truth about his motivation. That is, he cannot get out of this without either lying or "burdening" Natsume with his true feelings. Likewise, Natsume cannot satisfy Tanuma's needs without inviting Tanuma to fully join him in his world—which means facing one of his deepest fears. There is no space for half explanations or half solutions—either they deal with this fully or not at all. Either they remake themselves or they give up. That's what makes this the final crisis.
And everything leading up to this story supports it:
The Point of No Return. Ever since the end of the last crisis (Suspicious Visitor), the story has been building up Tanuma's connection to the exorcist world as well as his previously unexamined gifts. Since a big flaw in their relationship is Natsume trying to keep him squarely in the ordinary world, this sets up both the circumstances of the crisis and the resolution (i.e. the climax). Without this set up, Natsume allowing Tanuma into his full world would be too abrupt.
False High. The false high sets our expectations: Natsume and Tanuma should be at least shinyuu/best friends by the end, unless we're getting a bittersweet ending. It also marks an emotional high point and creates a false sense of security, increasing the dramatic effect of the crisis. Too close to the crisis and it's jarring. Too far, and it loses intensity. Honestly I should have seen this coming.
The Main Arc. The main arc revolves around Natsume's life mirroring Reiko's and how he tries to find a different path so he can live happily. More recent developments have been about finding Natsume's grandfather and thus learning the story about the end of her life: how she found someone and seemingly how she lost them and ended up a young single mother. The painting seems to be related to the grandfather—perhaps even painted by him—so finding it likely means finding her full story, which means we're close to that point in the main arc. This—as well as Tanuma—will probably have a significant role in resolving the Book of Friends issue itself. Considering how emotionally driven the main arc is, it makes sense to resolve Tanuma's arc first so that Natsume is properly supported for the main arc.
There's nothing else left. They're best friends. They support each other. They tell each other almost everything. They push their boundaries for each other. And it's not enough. If this isn't our last crisis, what would our last crisis be?
Now the, uh, "fun" part of this final crisis is that a lot of the time the main character decides it's not actually possible to resolve and they give up, leading to a physical or emotional "separation." After a bit of soul-searching and potentially some kind of outside influence, the hero changes their mind, transforms themselves, and comes back for the climax where everything gets fixed for real.
It takes two months for a single chapter to come out so needless to say I'm kind of hoping this "separation" part doesn't happen. Or at least if there is a separation it could be resolved by the end of this (likely very long) story. There is some evidence that the climax will occur in this arc, but it could also just be something that hints at a later climax.
Regardless, things are going to get a lot worse before they get better, because that's how this works. But the end result is in fact Tanuma's "happy ending," because "bittersweet" is just not what we've set up. I am completely convinced of that.
I'm still stressed though.