As far as i know, yes it is, although i have never personally been on a Sunday. As it is a border immigration office rather than just your run of the mill immigration like Suan Plu in Bangkok I would think it has to be open every day to allow border crossings into Myanmar (Burma).
Yes. Seven days a week, including Bank Holidays etc.
However, don't try to get stamped out after 4.30pm. The reason for that is that the Burmese side have closed up and so there's nowhere for you to go.
