First of all I am very sorry for the tragedy you have experienced. That would have been awful.
The "reaction" you describe to the Rompun was an intracarotid injection. It means the drug went into the artery (and straight to the brain) instead of the vein (so that it would go through the heart, rest of the body and be diluted when it gets to the brain). I am told that any vet who gives enough IV injections will eventually do this as it is easy to do based on the anatomy (the artery lies directly beneath the vein and the depth varies tremendously from horse to horse). This is why lay people should not take IV injecting lightly. It is a skill. I have seen 1 intracarotid injection so far. It is terrifying, emotional and makes you feel helpless because there isn't anything you can do except wait it out and hope they survive. The ones I've heared of with Rompun have survived but some other durgs are much less forgiving.
As for the tendon injury I have had good experiences with severed extensor tendons (front of the leg). Providing you keep the leg wrapped for support and keep the horse on stall rest long enough the tendon will scar down and the leg can be quite functional. The last one I did is back in work and on his way to a career as a performance horse. Keep your spirits up, he's got a chance to show again.
Best of luck.