Depending on what you are looking for in his mouth, you don't need it open all the way. I just slide her lower lip over to get a peek. This kinda sounds like normal BP, mine anytime I change anything at all she will pace and try every angle to see if there is an escape route.

It may be anxiety. You can try covering the sides with paper or the plastic aquarium sticky backgrounds that adhere to the outside of tanks.

One time when I had her at my breeders, she was roving around quite a bit and started huffing and breathing heavier. My breeder said my snake was kinda out of shape and chubby (what makes BPs soooo cute!) and that for her activity level it was normal.

That's all I know, hopefully someone else can offer more advice.