That's the fascinating thing about lava/magma's power to simply erase what was there before. Plate tectonics does a similar thing when continents scrape each other clean. For all we know, Earth had an advanced civilization on it way before us and we'd have zero ways of knowing about it.
Some bits of landmass have been around for a few billion years. From this we have atmospheric records that rule out a large industrial civilization.
I don't know. I'm unsure whether scientists do either. Chances are that it wasn't really any different than it is now. The resurfacing means that the surface is new - many fewer impact craters.
Venus' rotation is quite weird, could it be the result of an impact?