As far as the filter goes,I always vacuum out all lines of water including the heater.
I removed the sand filter drain plug on the lower side and left it off.
Last year,I removed plug,let it drain a day then moved it to one of my insulated garages,though I didn't keep it heated,so I have to guess any water at the lowest part of drain remained in there other than what may have evaporated out.I never did tip over to drain more out the top since I left the multi selection valve attached.
Still wondering though, after all I have done,this shouldn't of happened this quick if at the extend it did.