Agree with many of the post on this topic.
1st, you have a great trip planned. I hope you enjoy it.
Things to take:
A 6 plug extension cord, small cooler, lots of snacks, duct tape, small thing of shampoo, lots of $1's and 5's for diner tips and room attendant tips. I usually bring along some water or soda's. Lock's for your luggage. I like to bring an old style GPS with a plug to track where we are (put it in pedestrian mode and suction cup it to the window).
What to expect. On the Boston - New Orleans leg you will be in a Viewliner, so you will have your luggage with you (unless you check it) as there is a small luggage storage cubby hole up high. On the New Orleans to Seattle legs, you will be on a superliner and the luggage will be stored in an open area downstairs, so it is a good idea to have a small backpack or duffel bag to keep essentials in your car.
For this long of a trip, the diner car food will start to get old. Hopefully you have different menu's on the different legs, you can only eat so many Amtrak Burgers (which I think are very good). We tend to skip breakfast after a while and just have breakfast snacks in the room. One of the best parts of taking Amtrak is meeting new folks in the diner, so enjoy that.
Check your data plan on your phone, you will tend to use lots of data. The Crescent and Coast Starlight both have WiFi now, but I have found the WiFi on Amtrak to be intermediate at best.
Any station with an extended stop, get off and get some fresh air, meet some of your neighbors and make friends with the car attendant.
If you have never been in a roomette, your initial thought will be "wow it is really small". However, you will quickly get used to it.
I have no idea what they would do with singers or guitars in an observation car. Probably depends on how the other customers react. I will say that a guitar is going to be tricky to store on any of the trains you are on. I am guessing you would maybe be more welcome in the parlor car on the Coast Starlight than the other trains.
Speaking on the Coast Starlight (LAX - SEA), they serve lunch and diner in the parlor car that tends to be a little better than the diner. I highly recommend that. Make sure to get reservations early as it does fill up.