1. I agree with Andrea, but it's all a matter of time. Do you have the extra day to spare? Will it take a day from somewhere else? Judging by the available flights, a one-day visit would allow you to see only the Argentine side of the falls. It's your call!
2. Bariloche is a very attractive but relatively small lakeside town. nevertheless the surroundings are spectacular. Personally I would spend a minimum of two full days there. Here is a nice itinerary:
DAY 01: Bariloche
Flight BUE to BRC. Stay overnight.
DAY 02: Bariloche - Surroundings
In the morning and after breakfast start your tour of the surroundings of San Carlos de Bariloche city with the excursion to Circuito Chico. Visit Swiss Colony, Cerro Lopez, Villa Llao Llao, Cerro Otto and Lookout Point. In the afternoon visit Cerro Catedral (in winter, the country's main ski complex). Return to the hotel. Stay overnight.
DAY 03: Bariloche - Victoria Island and Arrayanes Forest - After breakfast go to Puerto Pa˝uelo, along scenic Av. Bustillo, and start a boat trip over the waters of Lake Nahuel Huapi to visit Victoria Island and the Enchanted Forest in Los Arrayanes National Park. Return to the hotel. Stay overnight.
DAY 04: Bariloche
After breakfast, off to the airport for the 10:20 flight to SCL.
3. Yes, it's possible to fly from BRC to SCL (Santiago). LAN has a direct flight, but it's not daily. Will you then come back to BUE? You may be able to get an "open-jaw" ticket with LAN, into BUE and out of SCL. Depending on time available, you might look into the famous lake crossing of the Andes from Bariloche to Puerto Montt, Chile, then fly on a daily domestic flight from there to SCL. If you do this, drop Day 2 from the above programme. http://www.enjoy-patagonia.org/bariloche.phphttp://www.limaytravel.com.ar/promo_i.php?id=38
4. If you ask Argentineans they will say Mendoza. If you ask Chileans they will say the Central Valley. To me it's six of one, half a dozen of the other. If you go to SCL, you can visit the best vineyards on a day excursion.
Hope this helps