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.
2) Yes
3) Conax package channels like digitalb @16*e and C+ @1*w can be opened with the Diablo.
4) Currently, no.
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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?