Is it permissible to instruct an elderly, healthy grandmother in her prayer from beginning to end, and is it permissible to compel her to bow and prostrate, and is ritual purity (ṭahārah) and ablution (wuḍūʾ) required for the instructor?

If the grandmother is of sound mind and understands what she is doing, then she must pray according to her ability, and there is no harm in your assisting her in that. It is not a condition for the helper to have ablution, but whatever touches her must be pure. However, if senility has caused her to lose her mind, then prayer is not obligatory for her due to the lifting of accountability, and you are not required to teach her or assist her.