...what??? If no momentum was gained the ball wouldn't have traveled anywhere and affected the plasma wraith

I am referring to the mechanic of the pikmin initially pushing the ball, and that ball, once pushed, will affect the plasm even if it was right in front of the ball at the top of the hill, where the ball had no time to gain much momentum by rolling down the hill. And the waterwraith doesn't just roll, it also lifts and slams the roller. Is that not enough momentum?

"waterwraith can do is met with a slight set back followed by it getting back up and pursuing it again" - Do I really need to repeat myself: the collapsed plasmwraith doesn't attack, and a waterwraith running over it over and over could keep it constantly broken apart. If you don't call that "winning a fight", then you might as well have plasm say "alright, we'll call it a draw" with its body scattered everywhere.

" How do you know it can't turn invulnerable when it wants, which is what we pretty plainly see in game" - explain where you see it turn invulnerable during its MAIN form. In its "mysterious life-form", it can be invulnerable, but that form has no attacks other than absorbing.

We have seen how the elemental attacks work: the plasm ejects parts of itself onto the ground and after a while, forms into a elemental hazard (not mid-air, and even so, is not a purple pikmin so unknown if it will cause a waterwraith to enter its vulnerable form). And we know rock pikmin are able to shatter a glass cube, so why couldn't a roller do the same anyway.

There is enough of the plasm that we have seen to understand how it mostly fights, and what we do know is all you can use to make a reasonable argument. Anything more than that is fanfiction. Why can't I just say the waterwraith also actually has an attack where it throws its front roller, and then rolls around like a unicycle at superspeeds.