First question --
For an older mare that has a tilted vulva --- this mare should have a thorough reproductive exam by a breeding specialist BEFORE she is bred. Chances are very good that she will already have an E-Coli or similar infection that will need to be treated at the same time she is sutured. Mares like this are easiest to breed AI while they are kept sutured. They should only be 'opened up' a few weeks before foaling so they do not tear. Then, they need to be re-sutured after foaling.
Are you going to be running barrels, cutting or doing other very strenuous performance training with her? If you are not, then you do not want to suture her.
When horses are sutured, a very small strip of the edge of the upper part of the vulva is cut from each side. Then, when they are sewn together, they actually grow together. Once a horse is sutured, it usually needs to be sutured for the rest of its life. There will be a small amount of scar tissue and the vulva will not close up like it did before suturing. So, do not do it unless a horse has a diagnosed problem, has poor conformation of the vulva or is doing strenuous work.
I have sutured old 'school' horses that were poorly conformed back there, and had them turn from tail-switching witches to pleasant lesson horses. This problem can not only cause infections and breeding unsoundness but can affect disposition.
In performance horses and race horses, I have seen mares throw their heads up and quit running or working right in the middle of a class or race. The instant their vaginal cavity fills up with air, they 'quit' their rider.