The pond de-iers usually turn themselves on automatically, through their built in thermostat, just above freezing, at about 34°F/35°F and turn off about 40°F.

So your de-icer will not come on until the ambient temperature surrounding it, drops down to around freezing.
Therefore you can safely plug it in now and you will be all ready when it does turn cold. Usually deep into the night, as we all know! And you don't want to be out there plugging in your heater, with the wind blowing into places where it really shouldn't go! LOL

I'm not in Zone 4, but I do cover my pond completely with a plastic tarp over a frame.
It keeps all the leaves and junk out and stops the cold wind from blowing over the water's surface. I have a plexiglass windoe fitted so I can see into the "tent" covering.

If you shield your de-icer with a board it helps a great deal to save on energy costs. Pond de-cers work more efficiently, if they are shielded from the winter winds.

You have cleaned your pump, so you can just store it as you suggested. There is no need to store a mag type pump in water over winter.
The old oil filled pumps, did have to be stored that way, because their seals dried out and then the pump leaked oil into the pond. Mg pumps do not have those types of seals.
"Horton"