I'm pretty sure "topit" is the root in verb. Topiti, should be the action of drowning, either self of someone. Potopiti, means that action was done, as with "potop" ( po-top, with "po" meaning "after" in slavic), or emphasizes the meaning of action. Of course there are other derivatives of the same root, but it's just the complication of live language.
Slavic names: I'm not sure if you got all of them right, but surely from the list of hundreds of Scythian names, there will be few coincidental. Also few slavic will show up too, as we know slavs where "citizens" of Scythia and Sarmatia (depending on time period). What I meant, generally speaking, is that majority of names are not Slavic. It means that in their origin and heritage Scythians or Sarmatians were not Slavs.
My conclusion was that, to explain persistence and uniformity of slavic language over vast territory, slavic needed to be a universal language of all the tribes that took part in slavic expansion. The territory that they conquered was vast, from Baltic Sea to the Mediterranean, from central europe to Ural Mountains (almost), that's half of the Europe. And they came from unknown place, and nobody heard of them before. That's like a miracle, lol.
Following this thought, I started leaning into direction of known political entity, to be a starting point, and fairly big one. Having a clue where the (topological) homeland of slavs could be, and some clues from history and legends, the homeland landed in area known as Sarmatia. Some other historical facts and Sarmatians names, lead me to conclusion, that Sarmatians where the ruling class only, over the Slavs. So at the end Sarmatians needed to be slavinized in order to achieve the cultural and linguistic unity of all Slavs before spreading around.
So even though we find only few Slavic names on list of Sarmatian names, even at the 4th century of CE, they were already Slavs. The most of the names are of royal families anyway. So even if they were Slavinized, they still used traditional royal Sarmatian names. You know, it's the tradition after all, and tradition is always strong and conservative.