Hi Denes,
I wouldn't be so scared of comparing the claims and facts, and I know well that you aren't either
After all that's one of the main points of history as a science. With the right sources and correct approach much of it can be cleared. No historical research can be 100% accurate and there will always be flaws and mistakes. Just like the wrong serial of Tomovich's B-17 that you found in my Bf 109 book (and this has nothing to do with sources but rather with fatigue). I'd say that Ivan took into consideration much of factors that you mention and took care to include them - total losses/damaged/crash-landed/belly-landed etc.
Cheers,
Boris