Your supposition is correct. Tuning the paired frequency will allow you to receive DME information from the TACAN.
If your DME tuning is through your VOR/ILS receiver, the VOR output will be flagged no computed data (NCD) as there's no VOR signal to receive.
The DME will tune the appropriate paired frequency and the DME should display normal data.
Having DME ...
It's almost certainly magnetic. Page 12 of the Chart Supplement (sample airport information) has this information at the bottom (emphasis mine):
All bearings and radials are magnetic unless otherwise specified. All
mileages are nautical unless otherwise noted.
All times are
Coordinated Universal Time (UTC) except as noted. All elevations are
in feet above/...
Yes, but (a) listen out for the 'Altitude cleared to' which will be given before departure since no 'top altitude' is published.
(b) Depending on traffic, you may also be given a different heading from the published HDG335 (= "or as assigned by ATC")
"Expect radar vectors to GBS" means ATC will, after identifying you, give you vectors, ...
You have a good grasp on departures so far.
This particular SID advises the pilot departing Runway 3/21 to turn heading 335° and expect Radar Vectors to the GBN VORTAC. This is normal procedure considering the fact that so many airports in the same area share SIDs. Not all airports will have a co-located Navaid from which to start a SID. In this case, ATC ...
I don't think you're supposed to just average the two together somehow by themselves. The two signals are designed to be used in different zones and in practice have very different signal strengths at any point on approach.
From EVALUATION OF INSTRUMENT LANDING SYSTEM DDM CALIBRATION ACCURACIES [sic] by Dennis M. McCollum [PDF]:
The course pattern ...