And if/when nothing comes of it, the left will just totally accept the job done by Trump's FBI?
i think most of them would. i'm sure there might be some voices that diverge, but i'm reasonably confident the mainstream left would not blame the fbi of shoddy work or accuse the fbi of having been directed to a political result.
that's not to say they would suddenly support kavanaugh - they opposed him based on his positions beforehand, and logically would continue to do so.
and if the investigation returned nothing to substantially corroborate or exculpate, it is likely that those on the left who currently feel that the unsubstantiated allegation is sufficient to cause prudent senators to vote against him would continue to feel that way. that too, would be logical - an investigation that does not provide new information would not lead persons to change their pre-investigation opinions.
we should definitely let the fbi investigate and find out, however. if it does turn out that a bunch of liberals attack the investigation, i'll happily join you in making fun of them.