Anyone have any recommendations for a good comprehensive map/guide to contemporary travels along Historic Route 66? I've seen several online, but don't know which is really worth buying and contains the maps and lodging/dining info I would want.
We've got several book recommendations and other guide ideas in the Route 66 page of the planning section.

Any additional advice or input related specifically to traveling the roads in west Kansas (Dodge City) or rural New Mexico (Old Fort Sumner) - how is cellular coverage in those areas?
Cell phone reception, especially in rural areas, is going to depend upon your provider. Even on the interstate coverage is good, but not 100%, but I certainly wouldn't plan a trip around cell phone coverage.

Anybody have any input on using broadband aircards for Internet access while on the road? What is their coverage and service like in the mid- and southwest?
Many motels, restaurants, coffee shops, truck stopes and libraries offer free wi-fi access.

is there a general average weight or percentage of space that trip baggage and necessities should be so I don't overburden my minivan (and put a significant dent in its fuel economy) on this long trip?
This thread has some thoughts of what people carry with them on the road. I really wouldn't worry about your fuel mileage. Unless you are carrying several hundred of pounds of gear, the difference really won't be noticeable.