Never listened to the Audinst before, can't help you there.
You should be fairly happy there. I can't say teh amp only T5 is, as a whole, better than E17. The SQ certainly isn't that big of a difference. You should be quite happy with E17, though not an upgrade, it isn't much of a downgrade either.
I have fairly big fingers too and I am fine. It is actually bigger than what it looks like in the picture.
D-zero is pretty much not a competition here. E17 is a class better, if not more. It has better amp, and better DAC section. Beside size and battery life, I can't see D-zero having any other advantage.