Enfo,

There is a pretty big difference in the seat, which when coupled with the fact that there are only like 15 business class seats, is why they go fast. You'll be ok in coach for that distance.

Yes, you can sit in any empty seat but for the business class section. Luggage can go in the overhead rack, which is quite roomy and can handle pretty much anything short of a foot locker. Additionally there is a floor mounted luggage rack at one end of the car, although that tends to fill up fast. With medium sized bags, I strongly recommend the overhead anyhow, since you can keep an eye on your things. That's not to suggest that there are lots of problems with theft, but occasionally someone grabs a bag that they think looks like theirs and really isn't.

Finally, I highly recommend that your parents not be late if at all possible. If you miss the train, you cannot board the next one with the same tickets. You will have to have new tickets issued and since it's not Amtrak's fault, you could end up paying more for the new seats if the prices are higher.

And yes, the trains do sell out sometimes.