Welcome on board askou.
#1:- You can check the quality of a DVD (after burning).
Two checks that are common.
TRT (TransferRateTest) - Look for smooth lines.
DiscQuality. Look for low PIE / PIF & Jitter.
Less common a check, but still good is Scan Disc. This tells you of any poor or bad sectors.
#2:- See answer #1.
#3:- Yes lower speeds not only give better quality of burn, but likely to aid longevity of said disc.
Too say nothing of possibly extending life of drive.
It is also true to burn too slow & the end result give a very poor reading.
As a rule of thumb. Most users will burn @ half the disc rated speed for reference backups.
All the above will work only if your drive supports the functions.
Your Plextor does not.
However you can use Plextools.
The scanning of discs using Plextools will take longer, but many believe the results to be more consistant.