Congrats! I'm actually an outgoing Diamond member after three wonderful years (unfortunately) but I'll answer your questions:

One coupon per person staying in the room. Free Continental breakfast only at the restaurant. Meaning you can't use the coupon for In-Room Dining or Express Breakfast. You could, however, order a full breakfast and use the breakfast coupon as payment, PLUS whatever cost remaining, plus tips. (the coupon actually have values in it) Check with your host/hostess, your mileage may vary.

Yes. Any Gold Passport member may use this service. It's one of the Hyatt's signature benefit unmatched by any other chain.

Yep. And before you go ballistic on the person behind the counter, remember that they're only humans and humans forget. I have stayed at a Hyatt for two straight years and at times they did forget to offer me an amenity (usually new employees), but a quick friendly reminder ("Can I get my amenity card?") before you leave the desk suffices.

Right. Upgrade to suite is based on availability. If the hotel isn't busy (Fri/Sat nights), you could get upgraded to either a Junior Suite or Real Suite, gratis. This is based on your Diamond status.

RC room is located on the same floor as the RC lounge (which, depending on which location are closed until further notice anyway - hence the breakfast coupons). RC rooms tend to have nicer bedspread, maybe more amenity in the bathroom (Q-Tips, cottonballs, magnifying mirror, scale, etc.) shoeshiner, but other than that it's the same size as the regular room.

Yes. Additionally, if the hotel is full and no rooms are available at the RC floor and no suites available, the hotel staff will try its best to put you in a corner room on a Gold Passport floor, where the room is obviously larger (sq-ftage) but the amenity is the same as a regular room. If the RC lounge is open, your room key will be coded for RC Floor access (most likely have to slide your card in the elevator). Yes you would still get your diamond amenity delivered.

My advice to you is keep your GP profile up to date on the Gold Passport website. If you had preferred a room close to the elevator, then you might not get corner rooms, which tend to be ways away from the elevator, depending on the floor layout.

You're just a curious one aren't ya??? I think most suites have king beds but I know a few RC rooms with double queen.

Before I close, let me share with you my biggest pet peeve. Diamonds who always insist on RC room or a suite. Folks, rooms are rooms. Unless you're on a lower floor where you can hear the traffic, it's a room. Try to think away from the airplane mentality where First/Business Class is better than Cattle. For the most part, almost every room in a Hyatt is similar in nature; has a comfy bed, a bathroom, a phone, a desk, a TV, mini booze, etc. (unless you're trying to impress your better half then forget what I just said).

Hope this helps.
Jamester