1.5 weeks for what you seem to want to cover is a good timeframe. You wont have a lot of down time, but you will also not be super rushed.

Iguazu-- one night is the minimum, some folks say you need two... but its a coins toss and ultimately depends on what sort of depth you like to cover things with.

Patagonia-- Im not a huge fan of Usuhuaia.... IMHO the place to go is El Calafate. Perhaps you should consider visiting both destinations.

AEP will be your hub for domestic travel.... its the only hub in Argentina (aside from EZE), which is unfortunate but its how it is.

September weather is very nice... its Spring. You may still get snow & cold in the south, especially where youre going but it will not be very harsh.

Please make sure to check out the various sticky threads on this forum as they contain a good amount of information on Buenos Aires.