Yes, you can get down. There are no ticket checks at Indian stations, nor is there any reason for them, since there are no crazy promotions that make it cheaper to travel 200 kms to destination X instead of 100 kms to destination Y.
Yes. You can get down in the middle. There are ticket checks at the exit sometimes (rarely though). But it is okay if you get down your train in the middle of your journey and you donot need to pay extra fares.
Yes. You can get down. I did this last month. My ticket was from Delhi to Kalka and I got down at Chandigarh. I was a part of the "random exit check" but they didnt create any fuss that says it was all legal.
You should inform the TTE/TC before doing so.
The reason is after you leave if any illegal activity occurs, then they can suspect you. Then it will be your responsibility that you are not attached to it.