Laurie, sorry to be short on answers about mag ox, and I'm not sure what your question is about flax.
But definitely add water to the beet pulp. Some horses like it soupy, some a little less wet. But feeding it dry can lead to choke....at least that was an unfortunate experience for me.
Iodized salt added to her bucket food is good. If it were me,wrt the block in her stall,I would use just a plain white one, or a white iodized block,if available.
You can keep adding to your case history after you load it. It doesn't have to be totally filled out . Just add the link to it to your signature,and give us a heads up to take a look.
Lorna in Eastern Ontario