Parenting anything to anything will have no effect at all on dynamic cloth simulation, they just use totally separate guts in Poser.

"Break through"  means that your cloth item is somehow being pushed farther than it should go, and in the case of the thong, that is from frame 1.  Collision Offset is 1.0 at default, and it often is not too safe to make it smaller; any time you have cloth snug against the body e.g. the thong, especially when it is pinched between two body parts like the glutes, there just isn't any point in making it dynamic, because you do not want it to move.  That is probably what is making your sim run so long and eventually fail, is the difficult math involved in trying to find a safe place to put that thong string (think of the immense pressure those powerful butt cheeks are putting on the poor thing, show it some mercy and don't make it dynamic ;)  )

The skirt is dropping away from the belt because the mesh of the whole figure is not welded together the way Cloth Room expects and needs it to be.  This is fine for non-dynamic (conforming) clothing, but for dynamic cloth, the mesh of an object has to be one continuous piece or it will fall apart like you're seeing.  You see the same thing with bras and other items when you try to convert them to dynamic cloth.  I do not know if you can clothify only part of your item, but if you can, then I would suggest only the skirt.

oops, ps:  to make the skirt stick to the hips, you can set a constrained group of a few points to keep it from slipping down:
http://www.renderosity.com/mod/forumpro/showthread.php?message_id=2844201&ebot_calc_page#message_2844201

My Freebies