If you have a horse that misbehaves at home and is difficult most of the time, it's wise to start with MagRestore. Many horses need a calmative due to a longstanding magnesium deficiency. Magnesium deficiency questionnaire http://www.performanceequineusa.com/magnesiumdeficiencyquestionaire.aspx this helps us evaluate your horse as an individual.
MagRestore would be recommended for horses showing signs of magnesium deficiency.
Typically what we see in working horses is chronic back/body pain in spite of excellent saddle fit. Difficulty with collection, stiffness, short attention span, poor work ethic, etc.
Loading dose: 2 scoops AM and PM for 10 days. Some horses with serious and longstanding deficiency may require a longer loading dose period. Maintenance doses vary from horse to horse. Most working horses require an additional 20 grams daily. 1 scoop AM and PM. Loading periods also vary depending on how deficient the horse is and for how long.
If you horse does not have these deficiency symptoms, then we recommend starting with Focus Equine

Yes.
For a horse that is generally a nervous type and worse at shows: We would recommend MagRestore to start. It's possible that this behavior is from low mag levels. This type of horse is also predisposed to low magnesium levels. We would also recommend Focus Equine for show situations to help overcome the anxiety, mitigate adrenaline (which wastes mag) and help to establish a positive association towards being shown, travel or clinics etc.

If most of the nervous behavior is resolved at home with magnesium, that's the best case scenario. If your horse still struggles with nerves, try Focus Equine as a maintenance dose at home to aid in overcoming anxiety.
For your horse that is currently rehabbing. The magnesium oil topically on the shoulder would be very therapeutic. Also very useful prior to any chiropractic or body work.
Also, it is very common for horses during and coming back from rehab to be low in magnesium. Although they are not working, I believe the stress of being injured, in pain and confined is very stressful and uses magnesium rapidly.
We have many clients with horses that prior to rehab, were good citizens, no issues to speak of. Coming off the rehab, they were hard to handle, chronically body sore, and showed signs of low magnesium both mentally and physically. These symptoms resolved with mag supplementation. Some needed ongoing supplementation, some were able to discontinue gradually until they were back to pre-rehab condition.

Our magnesium has no fillers or additives.

You can fill out our magnesium questionnaire.
http://www.performanceequineusa.com/magnesiumdeficiencyquestionaire.aspx

Usually blood tests are not a reflection of a horses true magnesium status. Only 1% of the body's magnesium is found in the blood, the rest is in bone or soft tissue.
You can also perform and EXATEST, which is non invasive, a cell scraping from inside the horses mouth. This gives an intracellular mineral assessment.