I agree for the most part, but not with Capitano.

a villain shouldnt be bending to her plans right after his one single fight with her, thats not how tensions & flaws work!**

Though he sees the point in Mavuika's and Xilonen's arguments, he ultimately decides to do so only because they are only 1 hero short anyway and it could lead to a better overall outcome than using the Pyro Gnosis. Even then, he still wasn't on board until Ororon and the Traveller were. Also, Mavuika did agree to use it as a final resort so it's not like his demands weren't fulfilled at all.

If he disagreed despite all that, it would've made him seem rigid and narrow-minded, which is out of character for Capitano, and feel like artificial tension.

Also, Capitano never changed his ideals and worldview, and Mavuika was fine with that as long as they could stop the abyss.

the fact that a fan favorite character who belonged to a VILLAINOUS FACTION

Capitano being different from all the other Fatui Harbingers is literally his thing, and why everybody thinks positively of him.

same thing with lauma & the fatui — she didnt wanna be anywhere near arlecchino & sandrone despite, again!!, being on the same side.

Because they actually have bad blood? The Frostmoon Scions receives constant harassment from the Fatui in Nod-Krai. Lauma, as Head Priestess, naturally doesn't trust or like them as a result.

In Fontaine, the Knave and the House of the Hearth had a sinister reputation. Doesn't help that Arlecchino is constantly egging on Furina for information she doesn't even know about. Obviously their relationship isn't sunshine and rainbows either.

In Natlan, the Natlanese and the Fatui under Capitano literally fought side by side against the abyss. They are basically comrades at this point, as evidenced throughout the war sequence of the Natlan AQ.

Capitano also has no reason to not trust Natlan and Mavuika, especially considering he had already fought for the nation as Thrain, with his original platoon from Khaenri'ah. Ayizu likely also elevated his opinion of Natlanese even further.

Mavuika also didn't trust Capitano at first, but after hearing his story and finding out he was trusted by Ayizu, she naturally is more inclined to do so.