1. Yes, you can sim a full season. Simming straight through the Preseason, Regular Season, and Postseason games without stopping to look at any box scores/stats/etc. takes roughly 45 seconds on my machine (quad-core, 8GB RAM.)
2. The computer does a very solid job of handling your team in the simmed season. It may make some decisions that you wouldn't make, but they all seem to have a logical base.
3. Career arcs are very, very good. The developer adjusts those with every release (and sometimes even with free yearly roster updates when there's not a new release in a given fall) to match current NFL trends.