I think she looks nice with her long mane...maybe just do a little 'maintanance trim' to get rid of any burnt, or split ends. I would do the same with the tail...only trim off what is necessary, to help get rid of the unhealthy ends.
As for the tuft of mane near the wither, on roached horses, it's to protect the top of the wither from excess rubbing where the saddle pad sits, atleast that's what I was told years ago, when my mom had a horse she roached all the time. I don't roach horses manes, so not sure if that's 'valid' or not...hahaha
Well, another reason for roaching, is that little tuft of hair will not get trained to the side with her mane! I have braided it over and everything, she rubs it out. I am thinking it would look terrible roached, but then again, her mane is really pretty. How long does it normally take for a horses mane to grow out?
That will depend on the horse, I think...how quickly does her mane grow now? Like if you trim the dead ends, how long does it take for that length to come back?
Realistically though, It could take a year to get her mane back to the length it is now...just depends on how well she grows hair I guess!
As far as training the mane/tuft, perhaps you could train the rest of her mane to fall on the same side as that peice. Try using some neck covers, and see what happens; I would probably band, or braid it to that side as well...all you can do is experiment. Personally for the showing you are doing, it really doesn't matter which side her mane is on...or if one peice is on the other side as the rest. I would prefer the tuft your mare has, in that spot, rather than in the middle of her mane!!! So it doesn't look too silly, in my opinion!