Do You Need Vitamin B12 To Utilize Folate
B12 is not necessary to utilize folate but it is necessary to replace B12 before repleting folate as otherwise you can induce subacute combined degeneration of the cord.  There are multiple mechanisms of folate absorption from the small bowel and large bowel which don't require B12.

Geographic tongue is not caused by B12 deficiency, and the actual cause is not known.  It does cause raw beef tongue.

You don't normally need to prescribe mega doses of folic acid with methotrexate.  Common doses are 5 mg per week, and 0.8 mg daily, though some physicians don't bother using folic acid at all.  Although folic acid was commonly prescribed because it was believed that methotrexate works by virtue of its ability to antagonize folic acid, it is now known that this is not true. It works as an adenosine antagonist, and so you can take the folic acid on the same day as the methotrexate if you wish.