I have a lionhead...I'll see if I can help.
How long does it take for their mane's to be fully grown?
When I got my lionhead he was 8 months old and already had a fully grown mane. I'm not sure if the time it takes for it to be fully grown is the same with all rabbits, it may be more of an individual thing?
How old does the rabbit have to be for her mane to start growing and showing?
Some lionheads are really fluffy even as young babies. Breeders can usually tell even when the babies are a few days old if they will have a mane or not by their fur pattern. The babies who will have manes start growing their wool when they are babies, although it takes a few weeks for it to get really long. But I've seen 8 week old lionheads who had an oveous mane, although it wasn't it's full length yet.
Does the mane stop growing when the rabbit is fully grown?
Um it doesn't keep growing in the sense that it will keep getting longer and longer forever. It does eventually reach a certain length and then stay that length. But I wanted to mention that I gave my lionhead hair cuts before and his mane always eventually grows long again. I've heard some people say that if they loose their mane (if it's cut or whatever) that it will never grow back, but that has not been my experience.
what's the best fruit, veg, to give them for treats?
They can have the same fruits and veggies that any other breed of rabbit can have...here's safe fruit/veggie lists...
My lionhead is the pickiest rabbit I've ever owned. He's a really picky eater! He hates carrots, which I think is funny since everyone always thinks rabbits like carrots. Try different foods and see what your rabbits like. Just make sure not to overdo it at one time, or you could give them stomach aches. Give foods that are high in sugar in limited amounts.
And just to give you a little more info about manes...
Depending on what genes they get, lionheads can have no manes, single manes, or double manes. It sounds like Rhino may have gotten the genes for no mane, if he never had one. Him loosing his fluffyness could have just been him loosing his baby fur. Domino, since she has a mane, has either a double or single mane and will probably always have it.