Question 1: You've answered this question accurately. It gets tricky figuring what is the CrewMax's actual configuration. Payload figures only apply to the base model of a trim line; the manufacturer doesn't subtract optional items from the payload and you must do this yourself. You'd like to find if a 'tow package' is optional or standard. If the CrewMax only
comes with a Class III/IV receiver AND the CrewMax has its own separate published specs, then you do have the full 1515# to play with. Probably add 150# to that hitch weight to account for up front weights of LP & weight distribution gear and the distributed weight of other personal loading inside the trailer. The manual will contain a diagram on how to actually weigh your hitch weight in the driveway (or see this
Question 2: I'd say the NCC takes dynamic road forces into account and is for the purpose of travel. That NCC is quite generous and shows they matched the axles & trailer nicely -- not always the case on the smallest new Airstreams; eg, 16 footers. You'll be able to put personal/family gear & food in the trailer and still carry water for convenience and to keep the center of gravity low.
Question 3: CSA? Canadian specs possibly. Canada has different requirements. We'll see what other members say. You'll not exceed this # if you camp like an average person; ie, not dinging up the inside of your Airstream with firewood, dirt bikes and other heavy gear.
Question 4: Tow capacity and the TV's (tow vehicle) GCWR are somewhat intertwined. Tongue weight must be 11-13% of the entire trailer weight for a stable tow with U.S. trailers. To not do this would be unstable; eg, lighten the tongue too much and it's like throwing an arrow backwards -- a trailer tries to switch ends with potentially disastrous results. I find that payload will be the first limit stretched by too large a tow; ie, if aiming to stay within TV specs you will top out on the TV's payload usually before approaching tow capacity. Tow capacity seems set more by the advertising department anyway -- you can only tow that much if you burden the TV with a 90# thoroughbred jockey, a quart of gas and nothing more.
There are other online sources for tow information. Don't have those references right in front of me. Airstream has a good start at their FAQ page