To the first point, the problem is that Poe largely gets most of the meaty character development, and gets the completion of his arc, whereas Finn's arc is basically a hard reset at the start, and seems to be aiming in a specific direction, only for Rose to "subvert those expectations" as was often said by taking away the logical closure point of that arc for some meaningless bullshit speech she gives him.
I disagree with this. I don't think Rose undercutting the end point in any way undoes his development. The development isn't fulfilled by him dying due to sacrfice, a death isn't what defines a hero, it is the decision to risk his life for others in the first place. And I think people tend to reduce Rose's actions as undercutting Finn's development, but that ignores that Rose is allowed to have agency too. Finn is a hero because he chooses to sacrifice himself, not because he dies. Rose comes around to see Finn as a hero because of his decisions and saves him because she believes it's right. Finn gets the development and the ability to carry that through for real in the next film.
I also don't think his arc was a hard reset because I think you are giving an overly generous interpretation of TFA to be addressed further below:
As for the latter point, I think to me it was largely about the role the character has in TFA. He takes on a leadership role with his know-how in the climax of that film, whereas Poe is basically just "cool hero soldier" and nothing more. Finn is the one who knows how to save the day and stop Starkiller Base, and risks everything to lead Han and Chewie there. One could argue that his primary motivation is to save his friend, but that's neither here nor there, and just speaks to what an innate good person he is. He grew as a character to go from being someone who runs away from a fight, to someone who does what needs to be done to save those he cares about. To me, that was enough to show that the plan was for him to become a leader in the Resistance throughout the films, at least until TLJ degrades him to actual comedic relief and resets his arc.
You say Finn takes on a leadership role with know-how and say that his primary motivation to save his friend is "neither here nor there", but that kind of glosses over this explicit dialogue:
Han Solo : Sanitation? Then how do you know how to disable the shields?
Finn : I don't. I'm just here to get Rey.
He does NOT know how to stop Starkiller base in any sort of substantive way. He is still demonstrating heroism because he goes to save his friend though. That is solid development. You are right that he goes from running away, to running toward the fight to save his friend.
I think it would have been reasonable to continue this story without distinguishing between loyalty to his friend and allegiance to the resistance as separate things, as you advocate. The development could have worked for both and it wouldn't have been that clunky for the sequel to take it from there.
However I think the decision to make the distinction between loyalty to friend and to a cause is also a valid one, and provides an avenue to explore the conflict from the ground with a more nuanced perspective. It doesn't reset his development. He isn't running away at the beginning of TLJ for just himself. He is doing it for his friend. He thinks if he stays that will doom her. I think in a very basic cost benefit analysis, it makes sense that he thinks it would be the best course of action. What could he offer by sticking around? This way he is at least helping his friend. No other option is laid out before him.
Crucially, I think it is worth noting that after Rose stops him and mentions the tracking, an option presents itself that could save the resistance. He now sees something that he
COULD do, something that only he can offer. He can actually contribute and he fully embraces that opportunity. The stuff that follows is just a further exploration on finding and embracing purpose. Could it be shorter or presented differently? Probably, but Rian wanted to explore themes of failure and use his adventure to show another angle on the universe and I think that is valid as well.
Not to say this was the most entertaining, satisfying, or efficient avenue, but I think there is more to appreciate than people reacting would allow themselves to critically evaluate.
