How high would it need to fly for the parachute to be effective?

Even with a rocket deployed parachute you need to be at least about 200ft above the ground for it to work. So there's a dead zone during takeoff and landing where you're too high to survive a crash but too low to parachute down. Military VTOL jets like the Harrier and F-35B address that risk using zero/zero ejection seats which can launch the pilot straight up high enough for a parachute to work but that's obviously not feasible for general aviation.

Since ejection wouldn't be feasible, wonder if an undercarriage airbag, perhaps like bumper rails to keep weight down and work around landing gear, would be effective in a crash?

If the aircraft is falling out of control there's no guarantee it will impact the ground with the undercarriage side down. Plus airbags would have to be huge and heavy to do any good.

If the wings are intact it will have gliding ability. In flight breakups are really rare, and the only other reason why I'd consider a parachute is total electrical failure while in meteorological conditions: i.e. can't see with my eyes or instruments. Night time in the middle of nowhere could also be such a case. If you can't see the ground you're probably gonna hit it faster than you want.

No it won't. Not with a wing there and canards of that size. Without power, that is a lawn dart. The only possible behavior of the rear wing is to point the nose straight at the ground in a ballistic trajectory for maximum speed of impact. That's a wide tail fin, not a wing.