nebulous

Haven't Breen kinda said ingame that otas are immortal?

I dont recall.

What I'm curious about is; can the remaining organic side of a human deteriorate so far that it renders the augmentations useless, or would the Union just deservice the unit because they needed too much to survive?

During the speech at the beginning of the game he says that "For the first time ever, as a specie, immortality is in our reach". It may be just propaganda but this would explain why the suppression field exists, other than to keep humans in check.