Okay, 3 seats are going to be a problem doing an al fresco move. They usually have part of the cars designated for families and large groups, there will be signs up if there are. Your option there would be to scout the car to see if there are that many seats open together in that section of the car you want just after La Junta, but it sounds like that would be really iffy given the passenger load.
As to crashing in the lounge, that happens enough that it has aquired a name here on AU: "Lounge Lizards". It depends on the crew, most do allow, but some don't. Those that do allow it will usually roust the Lounge Lizards out about the time the downstairs cafe opens in the morning (6 or 6:30).
Amtrak is very much YMMV and much, much less rule bound than air travel is now. Try it, again the worst that can happen is they will send you back to your seat. However, as a courtesy to other passengers who want to enjoy the lounge, you probably should shouldn't spread out over multiple seats/the floor in front of multiple seats until after 10 pm.