1- full fat to start with, and see how it goes, my children are 5 and 7 and still have full fat as they are quite slim and only have two glasses of milk a day. I think the guidelines is to go to semi skimmed when they turn 2, but you might want to ask your HV.
2- the HV advice (fairly recent) was to stop the bottle at 12 months ish, or phase it out gradually from 8 months. My opinion is that until 18 months is ok, as long as they don't fall asleep with the bottle in the mouth as it's bad for their teeth. And if they get used to having it in beaker as well.
3- Of course you can warm it up for a while, but it will be usefull in the long run to change to cold milk.