I put the mare back with her for the time being. I will feed them together for a couple of days. The mare weaned her a while back and won't let her get her head under there. The mare is due to foal again in 3 months. I'll keep you guys updated.
Be careful on switching feeds to frequently. I take at least a week to change feeds over. Adding a bit of the new one day than a bit more the next etc.
Since you have chosen the Omolene stick with it. I see no problem keeping the foal with the mare. Since she is bred back, she won't let her nurse now that she has started the process. If she does start to nurse again you will need to separate them again.