A non-universalist said that while they agree with you’re conclusion, they think the argument is bad. The preposition εν means much more than simply "in" (just see the discussions on εν in any lexicon, such as BDAG, LSJ, or Louw-Nida; or in a Grammar such as Wallace, Robinson, Porter, or Long). He also says "when" the preposition εν takes a dative, as if there's any other case that εν does take.

“So εν can mean a whole lot more than what this person suggests, and to claim otherwise demonstrates a lack of knowledge of Koine Greek. However, the locative sense (i.e. "in" or "among") is the most common usage of εν, and even when the preposition is used in an instrumental sense (i.e. "by means of" or "because of") it still likely carries an abstract locative sense as well (c.f. Porter, Idioms of the New Testament Greek). To be sure, had Paul wanted to communicate a purely causal/instrumental sense he had better options than εν.

For example he uses the preposition δια just one line before 15:22 when he says: διʼ ἀνθρώπου θάνατος, καὶ διʼ ἀνθρώπου ἀνάστασις νεκρῶν

"Through a man: Death, and through a man: resurrection of the dead." (My Translation)

Δια is much more strong in its sense of causation/means, and would have been a better choice. It also makes little sense for Paul to repeat himself in v. 21 and v. 22, and if we take εν as instrumental, that's essentially what he does. So I agree with Stanley Porter on this passage:”