The fundamental problem with Scar and Grian when it comes to forming alliances is a difference of expectations. Grian is a tactician. He assumes that in a war or a big multiplayer game, you need to get yourself a loyal team. Anybody who is on your team is your buddy, anybody who isn't on your team is your enemy. That is the only way to win. Anybody who won't pick a side is not to be trusted, and he is surprised and annoyed if someone won't make their allegiances clear.
Scar, on the other hand, is a diplomat. He never thinks about forming a team because his assumption is that everyone he meets is going to be nice to him. His charisma is high enough that this is true way more often than not. Even when he is being a chaos gremlin, he tends to get away with it for much longer than he probably ought to because he is funny and charming and just a little bit intimidating. He doesn't consider people he makes deals with to be teammates because he doesn't see the world in terms of teams, but as a network of shifting relationships. He is surprised and annoyed when the web starts getting sticky, with various friends starting to want him to fight other friends.
Grian believes that loyalty deserves loyalty, and that if you are someone's teammate you protect each other no matter how you feel. Scar believes that friendship is what it is, and that relationships can ebb and flow with time and circumstance. Note that in Third Life, newly-red Scar seeks to reestablish a friendship rather than confirm that Grian is still loyal to him, and that in Double Life, Grian justifies his secret soulmate by rationalizing that the relationship will in the end also benefit Scar by keeping them safe. Note also that in Last Life and Double Life Scar wanted to live alone but work together and Grian was not at all on board with that because you can trade and borrow and joke from separate bases but you can't really protect one another.
There is always going to be friction in their alliances because Scar wants a buddy and Grian wants a teammate and those two things are not always compatible at all. It's going to be the same thing with Hermitopia because the more Grian tries to loop Scar into the team, the harder Scar is going to fight to get away, while at the same time Scar is very unlikely to actually take sides against the Hermits because they are his friends. Neither of them can get what they want from each other because neither of them have figured out how to make the other understand what it actually is.
Maybe they need another panda sanctuary. I hear that's good for communication.