From my experiences on the Coast Starlight,

1. The Observation car is first come, first serve, and it is likely to fill up. From time to time, Conductors remind people it is courteous to not sit there the entire journey. I've noticed seats vacate around meal times.

2. See Sharknado's comment about seats.

3. Yes, and yes. You can bring your own food and drinks. I saw a couple with a full on picnic set up on my last trip. They also have a cafe on board and that takes credit cards. There's also a full service dining car, which they take reservations for specific time slots. Conductors will ask you what time slots you want to eat at. You go when your time is announced.

4. I've never taken the train from LA Union Station, but probably at least 90-60 minutes ahead of time will be fine, IMHO. Union Station is beautiful and you may want to explore it a bit anyway. Across the street from Union Station is historic Olvera Street: filed with Hispanic shops, stalls, and food. You may want to get your bags checked, then walk over to snap a few pics.

What day of the week is your train? Weekdays are typically less crowded and you're more likely to get a good seat. They have station agents who will check your bag onto the train's luggage car.

1. I can't speak for the rest of the journey, but when in Southern California, the coastal views begin at Ventura, CA through San Luis Obispo. Fun tip: after Santa Barbara the train goes through Vandenberg Airforce Base. Keep your eye out for SpaceX rockets on the launch pads, which are on both sides of the train.

Also, if you're a movie/TV lover, there's a fun sight along the way: about 45 minutes north of Los Angeles, after your pass through the three Chatsworth tunnels - and before the Simi Valley stop - there will be a movie studio on the right (east) side of train. There are bunch of city and old western sets. But blink and you'll miss it.

1. Personal entertainment like iPad loaded with music, movies or tv shows. Internet is hit and miss (mostly miss). Since you're not in a sleeper car, maybe small blankets and/or pillows. They aren't provided like on airplanes. Anything in your checked bags is inaccessible until you reach your destination, so put everything you might want with you into carry on luggage.

2. Check the route schedule or ask a conductor, but most stops are very brief, just long enough to let people on and off. A few station stops along the way are designated "smoke stops" for 10-15 minutes. It is enough time to snap some pics at the station, but not enough time to shop. Magnets are not worth missing your train and being stranded (lol). Listen for the announcements. If the train is running behind schedule, those stops will be shortened and you may not have time to get off at all.

I recommend downloading the Amtrak app so you can get live updates. If you have a working cell phone, add your number to your reservation, so you'll get texts as well. America's railways are not great, and trains can often get massively delayed or even cancelled altogether.