A couple of pointers:
You can rearrange the rendering order of slices and masks by dragging them. In your case, Skulpture you place the background slice on the bottom, then place a mask over it to mask out the area where your object is, then place the slice(s) with the warped content on top. Does that help?
4- So I made the mask (using a two masks and resizing them, etc) As you can not add bezier points to masks (add/remove, etc)
Double click on the line to add a point, or double click a point to remove it. This info is both in the video and the manual, btw.
I am presuming I need an 'invert' mask
I'm sorry, but for all intents and purposes, 'invert mask' amounts to exactly the same as making it a crop. Unless I'm really misunderstanding you here?
i saw that for crop but CAN we have multi crop for a layer or a comp
Yes, you can, just press 'add crop' multiple times.