Elemental priorities and ignore damage...

So, what did it do anyway? If i understood correctly, all absorb/immune/halve elements were counted as 1 in that moment, the others were 0, with XOR FF they become 0 and the others 1, right? then BIT combines the elements used in the attack with the accumulator, but i don't remember how... could you explain that to me? for the sake of my knowledge, you know...

By combining the elemental resistances and XORing with 0xFF, we get all the elements with no resistances. If we AND(what the BIT opcode does) the elements of the attack with the elements with no resistances, we get 0 unless there were any elements of the attack that had no resistance.

2) This one should be easy:
I found this line in damage formula of disassembly:
C2/0CC9: D0 57 BNE $0D22 (Branch if ignores defense)

Say i replace D0 57 with EA EA...
Will that mean that ALL the possible attacks(including Umaro relics, tools, throw on flying monster weapons, etc...) of FF6 won't be anymore able to ignore defense?

Yes and no.
Attacks set to ignore defense would no longer do so, but attacks that skip damage modification entirely would still not be subject to defense.