I played both PLA and the original game as throughoutly as possible, and while I know there's Primal Dialga from the Dungeon series I'm purposefully gonna ignore it since it's another canon entirely. Apologizes I reblog it's just to have more space to explain things rather than leave a big chunky comment or tag textwalls, so anyway
Does Volo launching a smaller scale plan make sense at all
I mean yeah, considering his name and whole character are based off the idea of Unstoppable and Unwielding Will Strenght, he absolutely would take any chances to reach his objective, and use any tool at his disposal to do so. Likely even attempting the same strategy he used, adjusted to better account for Random Time Travelers stopping his plans or legendaries turning their back on him.
[Previous statement] when presented with the opportunity (walking time anomaly)
So two things. As I just said, yeah he absolutely fucking would, but the use of who's essentially a living paradox is interesting to his canon. Peridot's existence (sorry if I'm sounding like an expert of her, I'm not I've just been reading all Diamoncard posts lmao so apologizes if I get some things wrong, as I only use Tumblr) in a way is not too dissimilar from the PLA player's existence, since they were too a character living in the present who got isekai'd in the past and caused HUGE events from which the canon timeline now kinda depends on, making the PLA player a living paradox too. The "only" difference being that Peridot wasn't Isekai'd but was born from one. So to Volo, she'd be like another case of PLA player (someone whose existence is already a paradox, a fallacy in the divine order of things), therefore a new person he can use to get to his goals. And it's better this time because of Peridot's visions, which would allow him to better claw his way through the divine system and potential reach Arceus as he could push things enough to call for Dialga's attention- and idk find a way to pull Dialga in and then use it (and Peridot) as bait for Arceus. Since I imagine Peridot is enough of a time paradox to call for Actual God's attention. (Unlike the PLA player, she wasn't purposefully summoned by him, so unless you specified otherwise I imagine Arceus has a close eye on her to make sure she doesn't wreck the order of things even more. Or have someone like Volo use her for that)
On top of that: canonically Volo did, or does eventually, get some kind of divine power. Because through pokemon ZA we know he's still alive so something happened/will happen to him that will allow him to like. Gain immortality or something, or get Iseakai'd himself (somehow) in the future. Either way it's certain he tempered with either Dialga or Arceus himself, which would give stronger reasons, narratively, for him to have used Peridot and her power for his own gain.
Can Dialga just puncture the small holes in the timeline
He is the god of time so I imagine so. Pokemon has canonically a multiverse, even just within the main games (just consider gen 7 as a whole), which implies the existence of multiple timelines he has control over. And as the god of time, while we don't have solid canon proof, we can imagine he has the power to cut and sew parts together to ensure time flows properly- hisolating paradoxical events in their own time bubbles or adjust them in a way that won't deform time too greatly. The kinda proof we have, all implicit, is the the Hisui presentation in ZA that shows how the player's events have affected but not destroyed the canon timeline- or in ScVi how the main player is implied (in the Indigo Disk dlc) to have been the one to give the Scarlet/Violet book to the professor through a time distortion and therefore have caused/encouraged the events of the actual game, again without the universe blowing up in an instant. I want to say he maybe only does it when necessary, but one can also assume he does this all the time and the mortals just don't realize.
I hope this was an exhaustive enough reply, if there's more doubts or discussions I'm down for more discussion regarding these games' canon, and if I got stuff wrong (in regards of Peridot) I apologize again. Hope this helps and have a good day man