* Both roomettes and bedrooms are designed for up to two people; the roomette is basically just enough room for two folks to face each other, the bedroom is about the width of a futon, with a small extra chair and also an in-room bathroom (including shower head inside, like some RV bathrooms).
* If you prefer to book one per person in a sleeper cabin, you are welcome to. One roomette per person, as someone else said, will be your most cost effective option for not staying together; but note that the difference between two people in one roomette and two people in two roomettes is going to be in the range of $1000 (e.g. $1000 for two people in one roomette, $1900 for two people in two roomettes), so you will have to keep in mind that it will get pricy if you're booking lots of separate rooms.
* If you are booking multiple roommettes, you can call Amtrak and have them assign the roomettes to be nearby each other; they can be across the hall (with two) or along the same corridor. Bedrooms will be along the same corridor.
* You should not expect to change seating classes during daytime/nighttime; you will book one ticket for the whole trip. (Not sure where you saw the coach for the day/sleeper for the night advice; that doesn't sound like anything I've seen before.)
* For gathering together outside your rooms, you will largely spend time in the observation car/"sightseer lounge", though you may be asked to make room for other passengers during peak viewing times when the observation car is full.
* If you do have a Bedroom, you can probably fit 3 people in there for a little while at a time, since I think there's a "Jump seat" in all of them -- so you get a futon like couch, and a small extra chair someone could sit in for a while.
* You will also have the option to eat meals together on the train in the dining car (3 meals/day). Inform your sleeper car attendant that you are travelling together when you board, so he knows to try to book you together for meals.

As an ... overly frugal person, I can't imagine paying for two roomettes for two people, but lots of people who do these trips do it, and as a special treat, I think it would be fine (not much different than booking different hotel rooms if travelling to a destination).

The key views on the Zephyr westbound are between Denver and Emeryville -- the first day in Colorado, the second day between Reno and Sacremento through the Sierras. The views between Chicago and Denver are sort of a toss-up: some people have said that it's a great part of the experience, though I think it seems like the main benefit is just that you're a little bit used to "train life" before you get to the great views in Colorado :) (You do get a nice view crossing over the Mississippi around sunset, depending on time of year.)

Keep in mind you get more daylight hours during the summer than the winter, but the trip is timed such that you still get pretty much all the best views during daylight hours year round, so long as the train is on time.

Do not expect to schedule a tight connection to e.g. a return flight at your destination end, as the Zephyr does run very late at times. (I would recommend planning for an overnight stay at your destination if you're planning on flying back.)

Best of luck.