Grown up? Dippy's still away isn't she? Where's Northern Lurker?
I understand the second dilemma, but don't know what to advise. Maybe if he's just broken up with someone it's a bit soon. Why were they on off and is it really off this time? The age difference isn't a big deal - my stepdad is a year younger than my mum - they were 32 and nearly 31 (and I was nearly 7) when they got married.
Dilemma A - think I'd say tell R. But again, does A anticipate that you will tell her or that you'll keep it all to yourself. Given that he seems to be a good dad to M and that you have a good relationship as M's parents though not together, you do need to think about the impact on your family that telling will have. Why is R thinking of breaking up with him anyway - is it based on suspicion or is she already a bit fed up over something else?
There, lots of questions and completely useless advice.