Mark would have gotten out eventually. Cecil knew that he was fighting Nightboy, and would’ve eventually sent the Guardians to figure out what happened when Mark didn’t return. I don’t see Nightboy as being able to evade the Guardians for long and Cecil would definitely have ways of making him capitulate, so I highly doubt Mark would’ve been in there for longer than a day or two. And while we don’t know how strong the Shadowverse denizens are, my guess is they aren’t strong enough to pose a challenge to a Viltrumite in that timeframe.