Can you use pvc and put a greenhouse over the whole thing? Will keep anything in there from freezing and you could leave the waterfall going though it does add to cooling of the water, but you probably don't get that cold in the daytime?
I covered mine last winter. We do have days on end of sub-freezing temps and the pond never froze and there wasn't ever even ice around the edges of the falls. I am going to bypass my falls this year.
If you water stays below 50 and therefore you are not feeding I think you could turn off the filters and have no ill effect, but you might want to drain the filters if they are above ground.
Mary