I had a 1985 75 horse merc,with the same problem. It is easily solved. Your remote control has a lever for neutral idle which doubles as a choke. If you bring that lever up before you start and then start it will work just fine. Let the engine run at medium rpm to warm it up and you should be alright. As for the engine stopping at very low rpm,mercury made those engines for racing ,not trolling. Your solution would be a smaller prop so you go slower at a slightly higher rpm. As for ethanol gas or regular gas,you will find that performancewise on this engine,it makes absolutely no difference except for paying an extra 40 cents per gallon.Just dump some seafoam carburetor cleaner/stabilizer in every third tank or so and you will do just fine,although it better for these engines to use 89 octane instead of 87. What may improve your engine performance is decarbing the engine. Buy a spray can of "seafoam" and follow the decarbing directions for 2 stroke engines.This improves compression and cleans out the carburetor during the process.
There is another thing that in this cold weather is very important and that it a high quality fuel hose and pump ball. The Walmart version turns very stiff and hard in cold weather,while a high quality ball remains easily workable.