I think the DOJ should appoint a sitting US Attorney, perhaps Huber, to handle this under a general power to delegate and not rely on the 1999 special counsel regulation to bring in an outside person like McCarthy or Giuliani.
I have doubts that the 1999 regulation was properly drafted and would sustain a well-argued legal challenge. I know this is inconceivable given that it was drafted by ueber-brilliant super-lawyer Neal Katyal, but there you have it.
The DOJ employs lots of ‘outside’ people in the various ‘task forces’ over the years. Their use is under the same kind of regulations that authorize a SC. Those regs would be VERY tough to challenge, and a successful challenge would have much further repercussions.