Hi BZB. Lovely to see you here. Things are much quieter on here nowadays.
I really haven't a clue re your buttercream, but what you mentioned certainly sounds enough to me. If you get it made a little before time, you could always wizz up some more. I always freeze my leftover buttercream, so if you did make too much you could freeze it. When I need it I let it thaw and give it a beat up by hand and it's fine.
Happy 70th to your Mum.