Sadly I have had the unfortunate experience of spending much more time on greyhounds than I would ever have wanted to, about 95 legs in the last ten years.

You better call the bus company and have your ticket changed if you want to get on one stop later, many times if you don't show up they will give your seat away to standby passengers and you will be screwed. Now, if you are lucky enough and your seat is still available, you will not be charged a fee unless that ticket would have cost more.

I'm not sure if I get your second question but I will take a shot at it. It would depend if you check luggage or load it yourself, your luggage if properly checked would go all the way through but that option is not even available on all routes, most of the time you would just load it yourself. Be careful.