If this allows an Adventurer to opt out of being the Monster until later, wouldn’t everyone usually pass, in the hope of being the final Monster (when most strategies have been played out and they know what Havoc score they need to beat to win)?

That’s a good reason to fail this proposal, and in its stead put… nothing, really. The ruleset works just fine if we want each Adventurer to encounter the dilemma “be a Monster now or never be a Monster; the only difference will be in your eventual Merit score”.

PS: I’ve changed the wording and placement of the proposed addition a little to avoid any weird side-effects.

*raises finger dramatically*
*drops it on the key*