The spikes in the TRT on the 20A4P do indicate a problem, but more likely a configuration problem that should be sorted out. Even through that configuration problem, we can see that the spikes are at regular intervals. The spikes in the reading curve also occur during the disc quality scan (notice the green line in the 16x scan, post #3), where the drive does not slow down at all ever without completely stopping & throwing an error, so the problem was not within the drive.
If there was a problem with the drive or the disc, the drive would have slowed down, and the spikes would have been more of a prolonged trough, the speed drops would have occurred at irregular intervals, or the line would have begun to get wavy and then tanked downward before recovering. The drive also possibly would not have reached 16x in the end. The Pioneer produced a fine TRT, which further supports the disc being fine. In this case, since seek tests were also done, I can tell you that really iffy discs usually cause failures on the seek tests.
As for evaluating the disc quality scans, you seem to have a pretty good grasp of what does look good and what doesn’t look good. The scans in posts 11 and 12 are well within limits and shouldn’t need to be re-written. There was an overall low PIF, overall low jitter (below 11% when reported by the 20A4P), and very little unexpected clumping of PIF.
The Disc Testing Methods and Software subforum may provide a little extra knowledge, as might browsing over scans in the Blank DVD Media Tests subforum to get an idea of what to expect from your particular discs.