Ferritin Regulation Problem. Previous Blood Test Showed Rbc Count Below Normal. Is Taking 1000 Mcg Safe?

I have a daughter that is 17 and runs distance races of 1 mile to 3 mile in HS. For about a year now she has continued to have probelms with regulating her Ferritin levels. Supplements are use to raise them, but once reduced the Ferritin leve drops off fairly quickly (in few months). In an April blood test her RBC count had dropped just below normal. I have read how low B12 (among other things) can impact RBC count. We have not had a B12 blood test ran yet, but with the season near year end I wanted to know if having her take a 1000 mcg would be okay in case it might help until we get a blood test in the near future. It appears that doing this for a short period of time would be unlikely to have any ill side affects.

In addition, a non-doctor athletic trainer recommended a Manganese B12 supplement at a high dosage, 100 mcg for two weeks, then 60 mcg for 3 weeks. I have read how high dosage or exposure to Manganese can have harmful side affects. Is this dosage high enough and/or long enough to cause adverse side affects?