Lots of questions and observations, in that post.
Yes, a firm grip is important. It does not give "more" energy, but allows what is there to work the action and not move the pistol in the hand or the arm.
Fiocchi ammo varies, period. There are purple, grey, and black box ammo from Fiocchi and soft and hard ball, it varies by lot or by year or "they" keep changing powders or something. Some works some does not.
PPU is "stronger" or hotter than most Fiocchi, and maybe is more consistent.
I don't think leaving the spring compressed changed it at all.
The Wolf spring rating and the "German" spring rating were obvoiusly done using different techniques, A 38 and a 50 lb spring are not the same- unless the measuring technique is different.
I have measured the three pack of Wolf springs before, and found them all to be essentially the same "weight"; in fact the spring labeled weakest, was stronger than the one labeled higher.
This one reason I say do not change springs unless you really need to, one does not know if a different spring will improve or make worse any particular condition. When you have handled a lot of lugers, you can tell by the strength needed to charge it, whether or not it has a strong or weak spring! Experience is the best teacher, keep on with your testing for sure.
You can measure diameters and count coils, and segregate springs for trial (and error) that way.
Whether you count 20.5 or 21 is not significant; are the ends ground flat? or left raw? this affects the over all length and coil count slightly.
Don't over think the spring - DWM could not decide which to use either. Sometimes the .30 and the 9mm were different, other times they decided the same spring was ok for either.
It ain't like flying a jet fighter.
Most of this spring "stuff" you just have to learn by doing, there is no way to explain all the interactions! JMHO.