So when cold it feels easy to roll over, but after you ran it, it provided enough resistance to describe it as "definitely not easy to turn over when warmed up"? Did it feel like it was hard to turn because of compression or as if something was binding? If you are so inclined, my next step would be to pull the plugs when cold and rotate it over by hand ( with a wrench on the crank nut) and get a good feel of how it rotates with no compression. Use this as a base line. Then i'd run it and pull the plugs again. Be careful not to burn yourself, use welding gloves or something, but pull all the plugs as fast as you can so the engine stays hot and try rotating it again. If it feels very reluctant to turn over when warm, even without plugs in it, that would seem to point towards mechanical binding due to expansion of something.
Do you have another distributor you could throw in to test the ignition theory? It sounds like your ignition is a trick set up, it may be defective. Its reasonable that it could fail in such a way that as power flows through it, it begins to malfunction. It could still be providing spark but going out of wack with when the spark is occurring. To test this i'd fire it up, if it starts up and sounds good shut it off immediately but then turn the key to "on" and leave it in the "on" position for however long you feel it takes for your issues to start occuring. After the determined time try firing it up again. if it runs but now runs rough, that would seem to indicate ignition issues after it's been on for some time.