I don't really see what's not to understand here:
- Fushi is building the houses out of himself so that he can fight knockers through contact with them. Just like the ship.
- Due to his detachment from human form and emotion to an extent but also desire to not see any more death he's become more belligerent and detached -> that's led to him coming off as kind of toxic and seeing any humans who try to fight themselves instead of letting him do it as suicidal and foolish where the point of this was to have him realize that he has to respect the other people's choices too, even if they may result in their deaths.
-They are not literally Immortals. They are skilled in some way with a skillset and mindset that causes Bon to assume their souls will stick around after death so Fushi can revive them constantly, and their skills will also be useful to Fushi along with them being extra 'lives' for him.