This is a Federal election. Can the DOJ get involved at this point, or just the GOP lawyers?
We don’t have a “federal election” we have 50 state managed elections.
For the US Senate the final arbiter is the rest of the Senate. Which can vote to seat whomever they dang well please. Rats stole a contested US house seat (in IN iirc ) by that mechanism 30 some years back. I don’t recall the GOP ever having both the chance and the b*lls to do the same in my lifetime, but maybe PDT will loan McConnell a spare pair.