Um no,then you misunderstood. If you only have one directv box being used as a source for both a local tv as well as the slingbox remote users; of course when one changes the other will be effected.

When using a decoder box (such as a cable box or satelllite receiver), the only way to have the slignbox work independantly is to use a seperate dedicated unit for the slignbox itself.

The exception to this rule is if you are using basic cable, expanded basic cable or OTA antenna feeds as the input spurce. The slingbox pro and slingbox pro HD models incoporate a built in tuner (BTW, only the slignbox pro HD has the ATSC HD tuner. In this senario, the slingbox cab be ran independantly without any additional equipment involved.

Yup, it's the same for comcast digital cable seeing that a cable box must be used.

BTW:
You cab use a slinglink or equivelant power line device if you do not wish to run a cat5 to the slingbox location.