1) You cant really compare these as they are so different, it depends on 'best for what?' The 6800 is best for High Definition, stability & user friendlyness, the 9100 is better on price. Neither will directly open more really, although the 6800 will get HD channels as a bonus but you are able to use a 9100 & internet to connect to cardservers which could give more overall. However, if you want ability to connect to cardservers then a dreambox or clone would be a far better buy imho, and cheaper.
3) Conax package channels like digitalb @16*e and C+ @1*w can be opened with the Diablo.
4) Currently, no.
Both receivers are an unusual choice in my opinion. You can beat the price on either models and get better equipment, but maybe you have a cheap offer?