Based on my limited experience redeeming CO miles, here are my answers:

1. Are the Northwest reward seats indicated on the wpaa site or nw site also available for onepass awards?
Not clear if award availability for CO onepass awards corresponds 1-to-1 with reward seats appearing on the NW site. Some people have stated NO, but it has been a 1-to-1 when I have checked award availability.

2. Do you need to point out award availabilty on NW or HP to the CO customer service reps? Do they hassle you over this?
YES, they always check CO availability first so it is wise to have them look at NW. No hassle, but they seem to try to steer you to CO flights rather than NW.

3. Do the CO cust. service reps check out CO routes first and do you have a choice of airline to pick for your reward? Does CO make you take a CO route if it is available over a similar NW route?
YES (see #2). No, you do not have to opt for CO over a similar NW route.

4. Does CO/NW/HP allow domestic upgrades on award travel on onepass awards?
Don't know for sure, but I believe you cannot upgrade award seats.