1. as soon as crossing and double slip switches are included, you can build your tracks as complex as you wish. this is not dependent on signals, of which there is only one kind.
2. yes.
3. maybe.
4. the game spans from 1850 til 2100. you can realistically use steam engines for about the first hundred years, which is a long time (if you don’t speed up time you will be playing 20-30 hours until you’ve reached the 1950s.)
5. nothing, you can continue playing.
6. probably.