Thanks for the helpful advice! I'd neglected to think that if I stayed on the Argentinian side, I'd cross the border many hours ahead of my flight to go see the Brazilian side before flying out...doh!
Re. flights: Iguazu will be the last stop on my trip, and before thinking too much about accommodations and such I snagged an award ex-IGU and onward to LAX via GRU, so that part can't be changed. And I'll be coming from Buenos Aires, so I'm pretty much "stuck" flying into IGR. Oh well, there are far worse problems to have, I'd say!