First off, quit trying to dry fit PVC. Even brand new PVC is not intended to be dry fit. The so called, PVC "glue" or "cement" is neither. It is a solvent that melts the top layer of PVC in both the fitting and the pipe. When the parts are slipped together, the melted surfaces fuse together and create a solvent weld. Second, you will have to cut out the flattened pieces until you get to round pipe. The thin wall is the pits, I have a system I installed in 1984 and that's what was used then. Fortunately, I buried it quite deep so I haven't had too many problems with breakage. I don't think you need to use a micrometer to test for roundness, eyeballing should do it.