I'm thinking she is going to get adjusted somehow. My guess would be ult rate charge speed. Right now it comes up really quickly for how large and potent it is.
I'll try to hit your questions in order.
Firstly, Blizzard does a great job on the Overwatch website (www.playoverwatch.com) of breaking down each character and showing off their abilities and then giving them a difficulty rating. Definitely check out that website, you will learn a lot.
You probably want to start with the easier heroes at first since you are still learning the maps and the game mechanics. I would recommend the one star difficult heroes at first (Soldier 76 is a good all round starter) but after a bit of playing you will probably identify a few that you enjoy more than others.
Again on playoverwatch.com there is a pretty good breakdown on the map and game types and what to do on each one. Once you get your feet wet you can study up on specifics but in general you want to figure out which role your character has and try to play that role the best you can.
If you are a tank, soak damage and protect your team mates. If you are support, keep people healed (unless, Symmetra, she does something different). If you are an attack or defense hero you need to focus on taking out the enemy supports.
Lastly, welcome to Overwatch and have fun!
