We bought our house ten plus years ago and every year I've been able to get the plug to thread onto the filter drain well enough to not leak despite there not being much left in the way of threads on the filter drain. This year I couldn't get it to catch at all.
I put a fernco fitting on the...
I would think that would do it. Expand it so it fits in really tight and hopefully it will come loose. I don't think there is any way you could damage it. You could also try a little saw and cut the duck almost all the way through and I bet it would break apart.
I'm in Albany so very similar weather and my CYA disappears every winter. Just see what your test shows next spring and go from there. By the way I have a good friend who lives in Livonia!
I'm not surprised to see those results. Your earlier post showed - TF-100: TA = 70, CH = 625, CYA = 80, CL = 4.5, PH = 7.2 so your chlorine is 1.5 ppm below minimum for a CYA of 80.
You can still have your pool party today. Just dose with liquid chlorine and brush thoroughly to mix up the water and you'll be okay. When my pump went I found a good deal on Craiglist.