My take on what may be going on

I think the GOP knows they need to ditch Kavanaugh or they are going to get hammered even harder in the midterms with women and they will have damaged goods with him in there for years. Maybe even bad enough to lose the Senate. They know they can't look like they are caving and giving into the Dems so they convinced Trump to do the investigation. They are probably aware that there is enough the FBI will find to make it look like they are doing the honorable thing by ditching him. Then they bring in a super right wing woman as quickly as possible to finish off the face lift.

Of course if the FBI can't find enough, then they can jam him through but I think that will hurt them in the midterms.