In terms of radius of curvature, how much deformation/bending of the access tube can be accommodated before problems will be encountered with the readings?

We recommend that you incorporate telescoping sections within the pipe. This will prevent buckling or curvature from occurring in the first place, so this question will never come up.

If you must calculate a radius value, consider the size of the probe, 5/8 inch in diameter by 8 inches long, and the inner diameter of the pipe you will use, which varies with the wall thickness of the pipe.