Your first few questions have already been answered, so I'll just touch on the last one. Getting good at a single role takes a lot of time. Sure you can get the hang of it after a few games, but the actually be good at it takes practice. When you do play these roles, don't just play around, actually focus on what you can improve on. If you are playing ADC, make sure you focus your energies on last hitting minions. If you are playing support, get to know your abilities (and who you are laning with) and focus on ways you can help your partner, harass the enemy, and be effective. One thing I will say though, a lot of it comes down to finding the right champion. Playing any role as a champion you don't enjoy or find fun kills it for everyone. So if you find someone who fits your play style, then play the **** out of them.
People like to throw around the phrase "practice makes perfect." Well that is utter bull****. Practice with purpose and focus, now that will make a difference. Just doing something repeatedly doesn't always helps, as much as doing something with the intent of improving.