To my mind, Mummer's Dragon means a false dragon. There's a bit in one of the books where Jorah (I think) talks about mummer's shows featuring a puppet dragon for the knight to defeat. If there is a real dragon buried in The Wall, it wouldn't be a 'mummer's dragon' at all. (personally, I think it's much more likely that Mel will tear the wall down in hopes of finding that dragon only to realize she's made a horrible mistake).
The way the prophecy pairs off 'sun's son' and 'mummers dragon,' it seems to me that it would be really weird for half of that pair to be metaphorical or, really, anything that's just a person. I think we can be pretty sure that 'the sun's sun' is Quentyn Martell, I'm of the opion that the pairs will be traveling together, or at least arriving together...
Pondering all of this, I have to keep reminding myself that we don't really know enough to figure this out... There's absolutely no reason for Martin not to be introducing a few new characters, for all we know Storm Crow the Crow Lord (with is cloak of 10,000 crow feathers) is bearing down on Dany, ready to offer his allegiance and that of his 10,000 crows. It's fun to speculate, but very likely fruitless in this case...
What do we suppose Dany is supposed to remember from the house of the undying? Is that something as simple as people who claim to seek your friendship might not always be forthright and trust worthy? Or is that reference to one the specific prophecies/visions Dany saw there? Maybe it's just an admonish to trust her Dragons, as they are the ones we saw through the warlock's subterfuge....