Thank you both! Using a cube fixed the second problem, plus playing with the collision offset and depth. The dress is still showing those horrible marks along the top fold, though, which I assume are self poke-through (they are not AO, as I switched that off). I'll play a bit more with those dials...

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)