The use of "will" in "If your car will not start tomorrow" is a bit different - it isn't future simple. It's an idiomatic use of "will" with a specific meaning. You could use the same structure in the present to talk about what is happening now, for example "I need to go to work now but my car won't start!" That means that however many times you try to start the car, it refuses to start. This works with other verbs too, for example, "I've told him a hundred times, but he just won't listen!" It implies some frustration with the situation and that you have tried many times. So, when you change the sentence to the conditional, "If your car will not start tomorrow", the use of "will" remains the same - it means that you should call a technician if you keep trying to start your car and it doesn't work. "They will not come tomorrow" is an ordinary case of future simple, so in the conditional it would follow the usual structure too. I hope this helps! :)

The normal way of saying this is "If your car does not start tomorrow . ." ". . won't start . ." is also correct, and which has implications as described by Joan S. NB. the contraction 'will not' > 'won't ' which is almost always used in informal spoken English.