In a reexcision of a moderately dysplastic nevi is it possible the margins could come back unclear still?

Possible, unlikely: "clear" or "unclear" margins are based on any microscopic changes in surrounding cells, so we can't tell by looking before we cut... But, when a re-excision is necessary, we usually take a bigger sample to have the best chance of removing all the surrounding skin that might have microscopic changes. So, it's fairly uncommon for the margins to still come back unclear after a re-excision.

Possible: In the majority of cases, when the appropriate margins are taken, the margins come back a clear. However positive margins do occur.