> Trump lacks the constitutional authority to approve his own nominee. <
Right. But suppose there is an Obama appointee in some important office. Director of XYZ, or whatever. Can’t Trump just fire that person, and appoint an acting director?
And if so, can that acting director be anyone, or must it be whoever was next in line at the XYX Agency?
By all appearances the FBI hates Trump. Replacing McCabe, via due process, with Cray is a normal procedure. Even so, the organization is probably opposed by the idea. So Trump has to weigh any show of force in getting his director in against the hard feelings it may engender. Trump may have deemed outright firing McCabe too politically inflammatory to serve his long term purpose.