Eroneous elevation & grade

You can't calibrate it, unfortunately. But the range you're seeing is pretty large. Mine was 1800 ft off this morning at the start of my ride, and that's unusually big. Normally if it's off, it only on the order of hundreds of feet off. Sounds like you might have a bad sensor. Mine shows a tendency to "heal" during the course of a ride, which makes me think that they're trying to use GPS elevation to calibrate it. Whatever they're doing, it doesn't work very well.

The only thing I've been able to do is to use the track from my Edge 800, which gets calibrated automatically. Kind of defeats the purpose of getting the VIRB Elite, but there's really no other choice.

As for the grade, are you talking about the grade shown in VIRB Edit? I have a suspicion that they're doing something silly like doing a fit to a series of points and taking the derivative rather than just using ascent/distance between adjacent track points. The grade wildly oscillates on the video.

I was checking the 910xt forum for a couple queries and think I have realized the error of my ways!

I have been riding with the Virb in its sealed waterproof case, I presume that this is screwing with the barometer - in my defense, I had thought that elevation was GPS determined.