Its the Zoacrystal which prevents aptom absorbing a zoalord, just like a guyver control metal stops him absorbing a guyver, though as shown with guyver 1 he can interface with the guyver and use it to enhance his attacks ala the freezing smasher. The only confirmed zoanoids with the aptom anti virus were neo zektole and enzyme 3, its unknown if he could absorb a libetus or if their super metrobolism would protect them.
No reason aptom as Sully said couldn't just steal a full zoacrystal and get that added into his body after all it worked for Waferdanos as he was never designed to work with a zoacrystal either
No reason a unit g would not work on aptom, the only thing it could prevent him from doing is keeping any more zoanoid dna he absorbs after he bioboosts since the current theory is once you put the armour on it puts you back to how your were when first bonded example being the scare on sho's arm still being there after the guyver regenerated him from scratch, same reason fans think if a female guyver got pregnaut she would have to avoid bio boosting until she delivers the child