Natural Abilities
This category includes abilities a creature has because of its physical nature. Natural abilities are those not otherwise designated as extraordinary, supernatural, or spell-like.
Relevant part is 'includes abilities.' Not 'includes everything.'
Everything is one of those kinds of abilities.
Wrong.
Every
ability is one of those kinds of abilities. There are things that exist which are not abilities. Like swords. What I am wondering is, is there anywhere that defines the form of a creature as an ability, or is it more like a sword.
There really shouldn't be any discussion about it
Correct, because Fusion is being ignored for this exercise because when you use it, it's easily capable of far outshining the general goal of the exercise while ignoring all of the other minor details.
because it is pretty clear as to what you get: everything that both creatures get. If both of you have abilities that overlap, you get the better of the two.
That's not clear at all. The dominant creature's form could show through, or the manifestor's, or it could be a blend, or a random mix, or even something unrelated triggered by the mixing of the blah blah blah. All are equally valid, because, unless form is an ability, the rules make absolutely no mention of how to determine the form of the fused creature.
And no, I'm not trying to make you mad.