Save for a few select match-ups, Duncan can't defend PF's anymore, so even though he's technically listed as the starting PF, that's really only true offensively.
As for Ayon, I don't buy that they're interested in him in addition to Baynes. Sure, they could potentially package Ayres with Daye and pay to dump them on the 76ers, but if they were really interested in Ayon, they either wouldn't have qualified Baynes, picked up Daye's option or both, to begin with.
The Spurs' offer is probably of the non-guaranteed variety. If he accepts and looks significantly better than Ayres/Baynes in training camp/preseason, then they'll make room for him.