So, Fireheart knew about the three, even helped bring Lionkit to Riverclan.
Do you think he actually disliked the plan? I mean, why wouldn't he dislike the idea of giving up most of his kits to their other parents?
He wouldn't be able to watch them grow, be a support if they needed it.
But also... I think he would disliek this idea because he knows plans liek these will bite you in the ass sooner or later. Why couldn't they speak with Sandstorm about this all, allow Leafpool to keep the kits, mayby lie to the rest of the clan about her adopting the kits and not being their bio mother, there are no rules Med Cats cant ADOPT, they just cant HAVE kits.
There is no way Fireheart didin't think there would be hell once the three and the clans find out about their parentage, he knew from Bluestar's situation just how AWFUL that is for both sides. Having their whole lives practically fucked up with just few words. Your loyalty questioned. And he knew from Yellowfang's perspective just how stupid of an idea it is to give your kits to others without knowing 100% they would love and care for the kit!
I think nothing exposes the fundamental flaws of the clans like forbidden kits. It's a pick your poison type situation. There are no plans he would've liked because the situation itself is shit.
You're arguing semantics. Half-clan relationships are also not explicitly condemned by the Warrior code. But the intention behind the rules is more important than the exact wording. Clan culture fills in the loopholes and ambiguity. Half-clan relationships are bad because it threatens the unquestioning loyalty a warrior is expected to have to their clan. Medicine cats are forbidden from having kits, biological or otherwise, because it distracts from their duties.
Remember that anything said in camp runs a high risk of him hearing it. He's very aware of every cat's spoken opinions. It's something he's used to his own benefit countless times but it also makes him intimately aware of Thunderclan's viewpoints. Despite being considered more outwardly "progressive", Thunderclan is not that different from the other clans. They just think they're morally above them (Fireheart is also self-righteous but I digress).
In Fireheart's experience, keeping the secret is the lesser poison. The last time cats were revealed to be half-clan all four of them were nearly publicly executed. One of them died. Years of loyalty and service failed to matter the moment it came down to blood and the code. And Leafpool's situation is objectively worse since it involves two medicine cats as well.
He's fully aware of what could happen, but ultimately just has to hope that the secret just doesn't come out. Crowfeather and Mothwing both have good reason to keep their mouths shut. They don't want to ruin their own lives after all. He doesn't know those two all that well, so he's just trusting Leafpool's judgement on them.
As for why he didn't tell Sandstar. Simply put, he doesn't trust her as much as she trusts him. Her being the leader of the clan doesn't help things. It's a hidden issue in their relationship that becomes a lot more apparent in hindsight after his death and the truth reveal. In the original plan of mega au, Fireheart is demoted after the truth reveal and their relationship doesn't survive the fallout. It's a whole thing.
I do want to make Fireheart's flaws more apparent in mega au, especially his tendency to do what he thinks is right regardless of consequence, and sometimes at the cost of his own relationships. He's very compassionate but not very empathetic. He never gets any less impulsive in his moral decisions, he just gets sneakier about acting on them.
This whole situation is the second time he experiences regret for not taking the leadership role when he had the chance. It wouldn't have fixed much but it would've been easier.
