tt/ss at least in Attic can descend from *tj, *ḱj, *kj, *kʷj (sometimes) or *dʰj, *ǵʰj, *gʰj, *gʷʰj (regularly)
I am not sure what is the mainstream view today but I think it has been said that it represented a [ts] sound of some short.
I don't know what the root of the word Moloss-oi would...