Was there ever a real chance of a Harry/Molly romance? Even leaving apart the age difference, the sheer inappropriateness of a romance between a wizard and his apprentice, and the Doom of Damocles, you'd think even Harry would know better than to risk being torn limb from limb by the only dark power that even he fears. I refer, of course, to Molly's mother, Charity. And Michael would let her do it.
The age difference ain't bad, especially given their potential life span. Molly's about 25 and Harry is 38 or 39, I believe.
Molly isn't his apprentice anymore, or if she still is, she won't be for much longer. Harry pointed out numerous times throughout GS that she was operating near or at full White Council levels. In regards to the Doom, I think both Harry and Molly are a bit beyond that, in regards to things they worry about.
As far as Charity goes...all bets are off!
Really, though, I don't know how she'd react. She has substantially warmed up to Harry in the last few books (although warmed up for her is a relative term). If they know what Molly's been up to lately, who knows, Charity and Michael could see Harry being someone who could get her back on track.