Exactly. Model A is Albert's backup system. But it cannot be Albert's soul if Albert is still around and the two are defying each other.
Ciel's Biometals, named for their respective Reploids, also have the abilities of said Reploids. What combat abilities does Albert have? He shields himself with Model W fragments, that's about it. Only the copy ability links them.
Consider what happened to the Big Four (red host eyes with Biometals' will overridden), Grey's red eye flash in the opening, and the fact that four broken capsules are in the same room as him. I don't think it's any small stretch that Model A's will was considered irrelevant to Albert's backup plan in the same manner that the Big Four's wills are irrelevant to their hosts. From Albert's view, the only point to Model A's soul is its combat abilities, which incidentally allows Albert's backup to realistically obtain enemy DNA data without Model W's aid.
So who better than the only copy-capable Class-S Hunter of 22XX?
Then we have the obvious design inspiration. Dual pistols, emphasis on aiming over firepower (albeit in a different execution, similar to the copy ability itself), rapid-fire Giga Attack, matching helmet frame and side extensions, the fragment itself carrying an even greater resemblance, and a child-like voice to boot. Similar character development in that it starts out quick to run but grows more aggressive and trigger-happy given an appropriate role model to follow. Model A is very obviously designed after Axl, and given the nature of Biometal (8 other models all referencing past characters), you'd need a hell of a good reason for it to not be Axl in the story.
Oh yeah, and even though this is english-exclusive, ZXA was not the first time we saw the name "A-Trans", either. It's also the name given to Axl's copy ability in X8.