1) You can pin prefolds to increase their bomb-proofness thus
2) You should only need to change as often as you should a disposable but many people tend to leave a disposable on for longer than they should anyway. How often you need to change your prefolds depends on whether or not your DC is a heavy wetter or not. If his or her bottom is relatively dry when you come to change then you are changing often enough, if you find the wrap is saturated and your baby's bottom is very wet then you aren't changing often enough. You have to get used to the idea that, unlike with a disposable, the nappy is going to feel wet when you change it. If you want to keep your baby's bottom drier then you can place a fleece liner over the top of the pad - works very well.
3) When the sun shines again hang them out on the line in the sunlight to get rid of stains.
4) Newborn pooey nappies just go straight in the dry pail in our house. For older children solids get shaken off into the toilet and rinsed with a flush if Icanbebothered need be. I never rinse them in the bath as the waste pipe of a bath is not designed for solids (especially smelly ones)
When you put them on, make sure that the edge near the top of their leg (does that make sense?) is secure. When i first started using reusables i kept leaving a gap by mistake! Same with the wrap as well.
Are you using liners and boosters? I have a liner in which (most) of the poo lands on and i can just flush away.
I leave dispoables on too long if i am honest so i do tend to change reusables more often than i would disposables(i only use reusables at home - just easier for me)
STains - wash with a pre wash and hang out in the sun to dry or if its not sunny (when will the sun ever shine again!) pop them on a maiden by a window that the sun comes thru.
A lot of people rinse off poo when they flush but i haven't done that yet and DS3 is 6 months old. i just bung it in a bucket. anything a bit more "solid" goes on teh liner anyway