OP I am so sorry about your losses :(
I too would be contacting a lawyer regarding the feed and taking it to court. there is no excuse why the company/Mill should advise that it be fed to horses.
I would also be contacting the feed store to tell them what happened so they don't recommend it to anybody else for their horses ! (if not already done)
*Member of the Quality Free-Choice Hay/Pasture Feeders Society* *In Favor of Turning horses out as long as Possible*