Gerhard,
"What do we know about Mediaeval European History? Very, very little"...
Well, that's what I thought when I studied it. But (for England at least) it may be as much as for the sixteenth or seventeenth centuries.
Parchment lasts a lot longer than paper. Of course documents got lost or destroyed (in revolutions, like the Dissolution of the Monasteries in England, or the French Revolution). To take the English records, we have in the Domesday Book a comprehensive list of the land-holders in 1086 and 1066. By the fourteenth century we have a lot of post-mortem inquisitions on property-holders, which often give a precise date of death; even earlier, we have a lot of wills and other transactions. By 1415, we have records which show which four knights were sharing a tent on the Agincourt campaign (because tents and their transport cost money, which was accounted for). Even with faked documents it is nowadays quite easy to work out what parts might have been genuine, or at least copied off another real document, on grounds of style.
"Bastards and cuckoos" exist in every society, and allegations of bastardy were common when the succession to ings was in question I can't see any real reason to suppose that theywere more common in the medieval period than now, especialy among the upper classes.
Medieval history has barely been scratched!
Mark