Yes, get some polyquat -- maybe 4 quarts -- and the DE. You may not need all the polyquat, but it will keep indefinitely, so long as sunlight doesn't damage the bottles. And it's safe and non-corrosive.

Add the DE and let it run overnight. Then begin dosing with small amounts of dichlor -- 2 ppm on your pool would be just under 1 pound, or about 1 pint dry measure each evening. Make SURE there are no other chemicals in the system -- tabs, feeders, injectors, etc -- and then add the dichlor directly to the skimmer, while the pump is running.

Test chlorine levels the following AM and in the PM, before adding another dose. Report your results. Also, notice the COLOR of the backwash water the first time you backwash.