My experiences are from late August this year, so take them at face value...

I can't tell you if Aerolineas (Austral) will enforce any weight limit. Like anything else, if it is a few kilos over, I don't think they would even worry about it. But like you said, it is cheap even if they do charge you.

Security at the national airports was very easy to manuever through. No shoes off and I didn't pull out my baggie once. The lines at AEP, USH, Iguazu and Calafate moved quickly.

If you are flying in the afternoon, there is a better than average chance that your flight will be delayed (in my experience) due to limited aircraft that seem to gradually fall behind schedule as the day went on.

I didn't call and reconfirm any of my flights, and had no issues whatsoever with any of my reservations. That may or may not be within your comfort factor. Be prepared to pay the airport tax before heading to security. They will stamp your boarding card.

Aeroparque is a pretty easy airport experience. There is a nice little restaurant airside where you can grab a bite and a Quilmes for a good price and wait for your flight.