Best argument for Shaw being there is that WE JUST SAW HIM DIE.
The Shaw we saw die and the one who appears as a ghost look very different.
Once again, if you think his ghost should have been scarred, then Obi-Wan’s ghost should be missing his lower half.
It’s clearly the same man.
The difference is that Obi-Wan wasn’t cut down until he died. Shaw’s ghost is appearing as someone who never existed.