Actually, you're wrong. In the first place, common-law marriage only exists in a limited number of jurisdictions--20% or so of US states, Ontario (plus something somewhat similar in Quebec and BC) , the UK (of course), some old English territories, and, oddly enough, Israel.
Second, "2 people living together and sharing payments"
is not sufficient for common-law status. In Colorado, for instance, common-law status only applies if the couple
describes themselves as husband and wife. I believe that is also true in most other US jurisdictions which recognize common-law marriage. Given that the MC 1) never calls Bethany his wife but 2) does say they were engaged--which is to say, he had
asked her to marry him but the marriage had never been formalized--it seems clear that they were not common-law spouses by any US standard. Given what I know of common-law marriage elsewhere in the world, it doesn't seem likely that they would have been common-law spouses anywhere else, either.
Third, you're assuming that "all property and finances are shared"--something which isn't even true in a great many
statutory marriages. We don't know if the MC and Bethany had one checking account or two, and we don't know whose name is on the title to--or listed with the insurance company as the primary driver for--which car.
Now, again, I believe from his earlier comment that
@Jeff Steel is a lawyer; I'm not, I don't play one on TV, and I didn't stay at a Holiday Inn Express last night, so he may well know something I don't, and presumably it's his knowledge which has shaped the plot. But on the evidence we have to date, there is no justification for
assuming common-law status for Bethany and the MC. (And even if they were common-law spouses, that doesn't necessarily give Bethany justification for seizing the MC's property; that would depend on the jurisdiction in which they live.)