Yep, ichthomal is what you're missing. After soaking the hoof, pack the entire hoof bottom with the smelly, sticky stuff. Duct tape the hoof and cover it, and then use a hoof boot to keep it all clean. The ichthomal will draw out the abscess is a few days. It's messy and you have to change it every soaking, but it works. After it bursts, just keep soaking twice a day in epsom until it starts to harden over.
There's a new product out there by 3M I think. It's a pad you put on the hoof to draw out the abscess. Same concept as ichthomal, but you only have to change it every few days and no soaking involved. I haven't tried it yet, but I hear it works.
For abscess prevention, assuming the debris is entering via nail holes, for $3 you can buy one of those wax rings that plummers use to install toilets. Put a little wax over each nail hole and that will help seal up any vulnerable areas. Reapply as necessary.