Last weekend, my 1992 300TE 4Matic Wagon, (137,000) was parked outside in Vermont and subjected to a -20 degree night. The next morning, when the weather was a balmy 5 degrees farneheight, the car refused to start. There was plenty of battery crank power, but the engine wouldn't catch. It killed me to keep cranking it that cold, (three tries and I'd let it sit for a few hours before trying again), feeling like I was scrubbing miles off the engine with each crank. When the car wouldn't start after several attempts, I waited until the next day. The second night was as cold as the first - well below zero, and the next day brought the same no start result. Finally, on the third day, with warmer weather, (teens), and a battery jump start from another car, she finally started on the third try.
My car does not have an engine/block/oil/or battery warmer, (another subject I know). It has 10w/40w dino oil and is usually garaged in Connecticut and never subjected to that kind of cold. It also just had a new head gasket installed with new valves, guides and stem seals, as well as a complete tune-up with new plugs, wires, distributor, etc. and has never had a starting problem before this weekend.
This morning though, when I started the car in the garage, (second try), the engine suddenly had a rough idle!? After driving it a while, when it had warmed up, it was still rough, but a bit better. It was the same when I started it this afternoon - rough when cold and still a bit rough when warm. This rough idle is new! Prior to this weekend the car always had a velvety smooth idle and started on the first crank. Now I think I must have damaged the engine by cranking it in the cold without it starting. (Though I REALLY hope I'm wrong.) Are these symptoms related or is it just coincidence? Any suggstions/experience will be welcome. I sure would like to get my old smooth idle and easy start back again. Thanks.

1962 MB 220SEb Fintail Sedan
1992 MB 300TE 4Matic Wagon
1995 Subaru Legacy AWD Wagon