If such a civilization was confined to the shores of what is now the black sea, but which before the current interglacial was a fresh water lake whose level was much lower than today's, then no, we wouldn't be able to detect it casually -- we'd have to work real hard to find it. For civilizations confined to open sea shores the situation is much less dramatic because the sea level rise there was less than at the black sea, but still, there could be some that are 30 meters under water (accessible) but completely covered in sediment (so inaccessible).

Seems... unlikely to me. Plausible, yes, but unlikely. In particular any pre-interglacial civilization would have had to develop during the preceding glacial period, not before it because that would take it back 100Ky, which is too much to be believable given what we know so far about our species' evolution (though anything from more than 100Kya would be hard to find indeed). And evidence of it would have to have been destroyed by sea level rise during the interglacial, but why would the civilization itself be destroyed by the interglacial?