hmmm maybe you could try and use a poiyomi shader and turn off backface culling , that should make the back side of faces invisible , or you could probably make a new shapekey in blender to fit your avatar more closely. Obviously this asset is trying to fit most avatars but some might prove more challenging than others.
ah unestablized crotch , we meet again, So ! to fix this we'll need to weight transfer your body's weight painting to the oyster , then armature link the pussy to your avatar , because whats happening here is that parts of your crotch are being manipulated by your thighs , and the oyster only references your hip bone , so what happens is your crotch moves away / into your pussy and the pussy only follows the hip bone.
if you want I can link a guide on how to do this or maybe you could reach out to me on discord and I can set up fix for your model with an appointment! @s14787 is my discord