Now its get a bit embarassing-FOR ME!
with all the advice and kind words from above i made a plan for today.
I totally went back to zero, starting at the 13 grain insert building up and at 1/16 cc again. I always made 4 rounds of 4fletched and 4unflethced arrows at a time and then paused for half an hour. I made drawings of every grouping. I went back to 15 then 18 meters, groups got bigger but more readable. I did NOT continue paper testing and loocking on the nocks.
AND MOSTLY i concentrated on every shot as if it would be the one big-buck-shot. And i tried the approach i received via pm, turned down the lights a bit so that i was not seeing exactly where my last arrow hit and i would not make mental adjustments but shoot consistent at the same point of aim (big problem for me).
The outcome:
Best groupings i had at 113 grain inserts, bareshafts and fletched ones together.
Seems like the weak indicating i had the whole time was partly because of me hurrying through the shots and not concentrating enough.
Then i shot the same setup out of the recurve, they went perfect, even more perfect than with the LB.
To confirm my findings i then started broadhead tuning with 4 x magnus 2 blades and 4 x field points. This indicated again perfect flight with the recurve but a slightly weak arrow for the longbow.
I then build the side plate out again to 1/8 past center and started all over with bareshafting again after the broadhead test indicated it was good like that.
Then i stopped, my arms get tired and i cant shoot consistent enough any more, concentration went down.
I think i am getting near now but will leave it until tomorrow.
Big Conclusion for me is: The main problem was placed behind the bow!
I am so thankfull about all youre help!
As it seems i will come to a positive ending sometime soon,
When it arrives i will test the AD from Terry and if this is the big solution i will have to order from the US. I checked all internetshops in Germany and as i thought, no one sells ADs, only CX, ICS, and mostly Easton and GoldTip and the one or other very expensive Grizzly Stick.
One thing i know for sure is after testing the arrow from Terry will get a place at my wall to always remind me that with kindness, generosity and help, you can make so much of a difference for people with problems.
Ill keep posting how it will go on tomorrow.