I agree AP isn't a set way of things or a means to an end - if you're doing it as a means to an end then that isn't AP, it's just co-sleeping or whatever.
Also I think it's okay to agree with AP principles for babies and toddlers because they make logical sense but not so much for older children, but it's about the way you handle it - it would be cruel for example to get to some set age, say 3.5, and switch suddenly from AP to supernanny techniques. IMO, anyway.
I think it's quite normal to find co-sleeping intensely irritating once you have stopped, I certainly do, and I loved co-sleeping back when he was a baby. DS is 4.2 and although he's got to the point now where I don't need to stay with him until he falls asleep, some nights he does want me to lie with him and depending on circumstance etc I will do. If he comes into my bed in the middle of the night I either end up putting a pillow between us so that he doesn't encroach on my space too much or moving him back once he's in a deep sleep. He can't share with me and DP any more - the bed is just too small.
If it helps, I was a single parent from when DS was 13 months. I have had both a casual-sex "relationship" and a proper romantic/serious relationship between then and now while DS was co-sleeping and this is how it worked:
With the casual sex thing, he would come over when DS was asleep or with his dad. If DS was asleep upstairs then we'd use the sofa bed or just the sofa
When DS was away, then we used the bed. Visits were quite few and far between but it was purely a physical thing so that wasn't really an issue.
When I met DP DS was about 2 - possibly just under 2? He still co-slept at that point. It took a while from meeting DP, realising I liked him, to us getting together, to sleeping together and then it getting serious enough for me to let him into my bed. All of this meant I had plenty of time to move DS out and get him used to that idea before it looked (to DS) like I was shoving him out of bed to make way for my new lover, because it wasn't like that at all.
Up until this point, again, if we wanted to have sex it was on the sofa/sofabed after I got DS to sleep or when DS was out of the house completely. DS did occasionally come into bed with me and DP once DP was sleeping over regularly but because I knew this was a possibility I had left it until I felt the relationship was serious enough to let DP sleep there. Before this if he stayed over, he had the sofabed and I'd sleep with him until DS woke up and then move upstairs, possibly come down again if I happened to still be awake by the time DS fell asleep, but usually not.
Waterproof mattress protector is good for accidents! Alternatively you could say she has to wear a nappy in your bed? That might put her off? I don't know if that's too mean
DS is (luckily) very good with nights so I haven't had that issue. I DO have a rule that if he comes into my bed he has to lie still and no fidgeting or kicking or he goes back to his own room and that works.