You've posited a theory, full of assumptions and question marks. It is not up to me to search out data for you. Every time I have pointed out evidence that contradicts your pronouncements on this thread, you have chosen to ignore it and shift your argument.
The facts are these: There were lots of things written on the continent during the 500s. No one, to my knowledge, speaks of the event you posit occurred in the 540's. I could posit that space aliens came down and flew the pyramids over Constantanople, and you could not prove it didn't happen. You can only show evidence to prove a positive.
So show your positive proof of a massive European depopulation due to a fimbulwinter.
Do you know what ONLY LARGE BRIDGE means?
For one thing, no one around to use a bridge, or to build it. Secondly, they used wood so we can date these things with considerable precision through the science of dendrochronology.
http://www.morien-institute.org/darkages.html is a very colorful Welsh oriented site.