[mycroft]

The issue is not that "South should have found a force to game", it is that, given the explanation and their agreements, she didn't have a call that was an unambiguous force to game, because their agreements didn't cover the explanation. So she made a (reasonable, but probably wrong) pass, "knowing" that she'd get another chance to bid. And she did, and she did. And she didn't get to game because the pass led North to believe that South's hand was weaker than it was - but, however bad the decision to pass was, without the MI, she wouldn't have made the "ambiguous meaning" problem that caused her to pass.

I don't think that "I could bid <one thing>, but partner might think it means <something I don't have>, if he thinks that <other defence> applies, or <other thing>, but partner might think...if he thinks <defence> applies; or I could pass and hope to recover knowing I'm getting another bid, even though partner is going to take me for weaker." is necessarily "serious error" territory.

I don't say that OP made a wrong or bad decision - this is definitely a "listen to what she says, and rule accordingly" situation - but it isn't necessarily a "serious error unrelated to the infraction", in my mind, to pass and hope to recover when all calls could be misinterpreted.