Every act of the feds is a Constitutional matter. You should know that.
The issue is reasonable suspicion or probable cause. There is none. Thus no constitutional grounds for federal action per Amendment IV.
Don’t need to go any further than that, but a far as conflict of interest goes, there is reasonably a conflict of interest in Mueller.
I wasn’t suggesting that everything Mueller has done would meet constitutional muster. My point is that the mere existence of a “special counsel” is not in and of itself unconstitutional.