@nmstough: After reading of your experience and @knighti10’s response elsewhere on this forum, I was fully prepared to dig out (and undermine) all of the plumbing below the downpipe to give it room to move. Given the stiffness of your plumbing, that (or calling a plumber) might be your only option.
If you read my post (and the conversation with @tmcgahey), you can see that I definitely had some give in my plumbing. I could easily move the upper pipe out of the way to make enough room to slip the sensor with both SharkBite fittings onto the bottom pipe. My pipes did not come apart or go back together in a perfectly parallel, up-and-down manner, but I could move/twist/bend things enough to align the upper pipe with the top of the upper SharkBite fitting, from where it easily slid together. The pipe and fitting were slightly misaligned as they first came together, but they straightened out with each other as the joint slid closed.