Things like telepathy etc don't work on Deadman, IIRC.
So psychic energy, emanating as it does from living creatures, wouldn't affect him.
Whereas he can affect others.
So if there was a body (some poor innocent random), Proteus could take over first,and then Deadman would just supercede him.
Or Deadman takes possession first,but Proteus wouldn't be able to then take over.
proteus is undoubtedly the more powerful. he was also able to possess multiple bodies at one time. could deadman possess proteus? i doubt it. proteus was able to split his consciousness into several pieces. even if he did get proteus' body, he wouldn't get 'proteus' himself since he'd just...move his consciousness. or maybe his multiple consciousnesses would just overwhelm d-man and give him the boot? hard to say. not sure exactly what has affected d-man in the past so hard to say just what proteus could do to him directly, but i can't see d-man doing much more than annoying proteus. /shrug
Deadman can split himself up to possess different bodies.
OP has only a single body. Once Proteus takes possession of it (am not going to go into who is faster at possession lol) and starts expending energy.....all D has to do is wait him out and burn him out.
D has possessed Swamp Thing,and gone into the multiversal Green.
if it's not a battle of possession, (not sure who would take it if it were) proteus wouldn't need the body anyway. he'd just destroy it. /shrug
then d-man would need to try and possess proteus and i have no idea if he could. i know he's been ousted a few times. could proteus's reality control harm him? i don't know. i'd need to see the times he failed (iirc phantom stranger threw him out once, but can't recall who else has) to get a better idea if he could take possession of proteus. maybe. and wasn't there something wonky with ST when d-man took control over him? his consciousness was gone or something....? thought i remembered someone bringing that feat up before...