I went the Edwards route at R3 and was hoping for CCJ to give some level of ruck cover on the run home but all that was shattered in one game on Friday night. May not see CCJ again this season - calf injury or no calf injury.
Marshall is certainly an option but is probably a little more SC relevant than DT. Also his foot has been a recurring issue although seems ok atm. Bringing in these guys who have had multiple setbacks during a single season can be a risky business as Neale owners just observed firsthand.
The upside is he's fairly cheap now and can provide ruck cover. However given all your trades I'm not sure you have to worry all that much about ruck cover. If either Grundy or Gawn misses from here I'd just swap them out for Darcy with just 7 games left.