Hello Jooney, You are correct in understanding that 'might have' usually has the meaning that some event is possible. In sentence B "You might have met him if you'd been there" also implies that it would've been possible to have met him if you'd been there, but then again you just as likely might not have met him. In this meaning the speaker is not placing any judgement on the fact that the person being spoken to was not in attendance, it is a simple and passive statement regarding what was possible.
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jooneyWhen you change 'might have' to 'would have'These are different.