The reason horses bleed is usually because they are gasping so hard for breath that they are actually bursting some of the small bloodvessels in their lungs. Giving a vasodialator makes it easier for them do breathe by allowing the blood vessels to have more surface area in the lungs. This means they absorb more oxygen with each breath so they don't have to breathe as hard to get the same amount of air. It's essentially the same concept as Afrin nasal spray for people. Afrin works by dilation the blood vessels in your nose, making it seem easier to breathe. A horse that still bleeds on Ace would probably bleed a little more, but because the bloodpressure in the dilated blood vessels is slightly lower they may not.

Lasix helps these horses in a different way, by removing the fluid buildup left as a result of the inflammation from chronic bleeding. It helps to ensure there is no fluid in the lungs that may potentially block oxygen absorption.

And no, Rompun and Demosedan are not controlled, because there is really no potential for human abuse. The reason Ketamine is usually used prior to surgery is because it can have a very rough recovery, and a 1000lb animal panicing and thrashing is not a good thing. When used as a preanesthetic it wears off before the patient wakes up, which eliminates the recovery period from it.