Someone else will be able to answer the rest of your questions, but I would definitely try to get a seat reservation. Those trains can be absolutely packed - as in you stand shoulder-to-shoulder and can't move. It's not a great way to travel for the few hours it takes between the two cities. It's easy to buy a ticket at the station, but it's no so easy to get a seat reservation on short notice.

Yes, a conductor comes through at some point and punches your ticket. Use the restroom before you get on the train. They tend to be beyond filthy.