An acceptance email should be short and unambiguous: accept in the first line, restate the terms in the second, ask about onboarding in the third. Reply on the offer email thread so the whole record sits in one place.
Most employers ask for a signed offer letter, but a written acceptance email alongside it creates a clear timestamped record of the terms as you understood them.
Yes. It costs one sentence and catches discrepancies between what was discussed and what was documented, while the offer is still easy to correct.
Practically, no. Negotiate before you accept — once you have accepted in writing your leverage is largely gone.