Hi, Sanguan,

You can check availability for most classes for revenue seats of all the carriers you cited at:
www.itn.net

But award seats are a different story.

All I know is ....

On NW, economy is W, as you say, business is D, which is also now used for upgrade inventory from economy using miles. They can be checked at NW's web site. As far as I am aware, they are not shown on the above URL.

On UA, XY, XC and XF are award seats. NC and NF are upgrade inventory for business and first classes respectively. But they are not shown on ITN, either.

On AA, C is used for upgrade inventory from eonomy on internaitonal flights. A is upgrade from domestic econ. I heard A is sometimes shown on ITN but I myself never noticed it.

That's all I know.

Now, I can access AA's C as well as NW's D and W, via JL's Axess booking system but this is available perhaps only in Japan.

Sorry, not much of help.