No need to run cable from computer to the Roku DVP if you have a wireless network -- it has wireless built-in. The Roku DVP should plug directly in to the TV if possible. If the TV does not have any more inputs, you'll need some sort of switch. I don't think the Roku DVP will like you trying to "pass it thru" your satellite STB.
Tina said that she had wireless -I'm not sure if that meant mom does too.
A switch may be needed if either the sat-box or something else is using the RCA inputs (and there's only one set) - the sat-box might connect with coax.
The Roku would likely have problems routing through a sat-box.