it is impossible to cut a straight mitre on warped skirting board,take it back and get some flat stuff. as for the rest it would take pages to describe in full and years of faffing it up to learn to do it well.
1) your new plaster probably has what we call in the trade a bell bottom ,put a small straight edge against the wall and see how much the plaster kicks out at floor level.Hack of the worst not any higher than about 2/3 of skirting height.
2) where mitres join pack the back of the thinner board so it comes flush at the front.
3) onlt straight joints and external corners are mitres internal corners use a scribe,
4) yes mitre behind pipe if you cant get a full length in,
and if thats no use find a good joiner 🙂