It sounds like the shower's (and tub's) waste water is causing the shower trap (if any) to siphon dry, while at the same time pressurizing the system downstream, forcing gas out the now-dry trap. This can only happen if there's ineffective (or no) venting downstream from the shower trap. I'm having a hard time imagining how I would construct such a system, but most models involve a missing, misplaced, or failed vent somewhere.
The fact that draining the tub also siphons the shower trap and causes the same problem also strongly suggests there's a missing or plugged vent somewhere. When you drain the tub and get the smell in the shower, does the smell ever stop? Seems it would continue until the shower trap refills.
I would continue to try to contact the previous owners in an effort to find out how long the problem existed, although no previous owner in his right mind would say anything but "Nope, never had such a problem", since the penalties for failing to disclose known defects are severe.
I'd continue checking with the contractor and local permitting agency to find a set of plans, and the local plumbers to see if anyone knows of the problem. See if your camera-guy can construct a diagram of the system based on what his camera sees.
As a temporary measure, or as a last-resort fix, check out:
http://www.trapguard.com/
which is a little flapper device to close off the drain unless water is flowing out. Don't know if it will fit/work in the shower.