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.
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.