In all of this with the sleepover issue it isn't clear what YOU want. There's a lot of talk about what your wife and your girlfriend want, but what is your ideal? There is probably some kind of compromise between no sleepovers at all and numerous sleepovers a week.
As for the issue of your gf not wanting to meet your wife, is she comfortable with you having a wife or would she rather have you all to herself? If it is the latter you're in for issues down the line because that line of thinking ususally ends up in "it's either her or me". If she doesn't want to meet for other reasons it's probably best to just leave it alone and let it develop organically. Maybe they will meet one day and maybe they won't. Maybe they'll get along and maybe they won't. Just because you like her doesn't mean your wife will, and just because you like your wife doesn't mean your gf will.
Metamour relationships are entirely unique. They alse vary quite significantly. None of them are "normal". Let it be what it is (or isn't) between the two of them. Their interaction is beyond your control. You're going to have much less stress if you just let it go.
Everything will be ok in the end. If it's not ok it's not the end.