If you miss a train for which you reserved a spot can you still use that ticket for the next one?

I have done it many times. As long as you are departing from the originally scheduled station and going to the same destination, the time/date doesn't really matter.

Yes

I've done it about 5 times, it's never been a problem at all.

Usually. Call 1-800 USA RAIL. They can better assist you.