Fraktal

If you're using KER, it's roughly the mass with which you'll get 3400 m/s of dV if your sea-level launch TWR is at least 1.4.
Any lower dV and you won't reach orbit. Any lower TWR and you'll lose too much dV to pushing against gravity. For an upper stage, you can get away with less TWR but too low and you won't be able to circularize in time. That's really the best I can say.

There's no getting around researching fuel ducts, I'm afraid. You need to research Fuel Systems (ie. the tech tree node that gives you the fuel duct part) before you can crossfeed decouplers.

You may want to consider getting the Trajectories mod. It gives your map view a visual prediction of your actual trajectory during reentry and approximate location for landing, based on your craft's drag, lift, AoA and how much Kerbin will rotate under you while you're coming down.
Once you have it, it's merely a matter of setting up a maneuver node for deorbiting and tweaking it until the X that indicates where you'll land is near the runway. Then it's just a matter of putting the shuttle down on the ground once you're past reentry.

You can do this with the CommNet Constellation mod. If you're not familiar with it, it allows you to set different frequencies for your antennas to use. Two antennas can only talk to each other if they're using the same frequency (everything uses frequency 0 by default) and relay satellites only relay between different frequencies if they have an antenna for each frequency. DSNs can talk to any frequency, but you can disable specific frequencies so that the DSN won't talk to antennas on those frequencies directly, only through a relay.
Create a new frequency for the rover's use. Set the rover's antenna to the frequency you created. Set the DSN to not talk to the rover's frequency. You'll have to do it for the entire DSN, not just the KSC. Set at least one antenna on the relay satellite to the rover's frequency. Set the rest of the relay satellite's antennas to frequency 0 (or any other frequency the DSN is talking to). This way the rover won't talk to the DSN directly whatsoever, even if Kerbin is visible and in range. It will only talk to Kerbin via the relay. Keep in mind, however, that your signal strength won't be as high via the relay because multiple antennas on the same vessel only combine their power if they're on the same frequency and you'll need at least one antenna to communicate with the rover instead of the DSN.