There appears to be quite a disagrparity between various sources I am gathering about Kannalkampf
So this issue requires some deeper analysis with non british sources prefferably included . (I don't mean conspiracy theories ,but honest mistakes do happen)
Wood and Dempster used the Luftwaffe quartermasters records for thier loss figures, I assume Cooksley did the same. There are no British records of German losses, just records of British claims.
a) If the proportion of LW to RAF losses was the same as in the whole BoB then why did they withdraw from the channel so hastily? (leaving their convoys to the mercy of LW)
They didn't.
The convoys were reduced for a period because Dowding, quite naturally, wanted to concentrate air cover over his airfields, rather than have extra targets to protect. But the convoys didn't stop altogether. And the convoys were reduced, not just left unescorted to the mercy of the Luftwaffe.
b) If the LW could provide protection for the 1900 slow barges to be successfuly moved through the channel to the French ports why wouldn't they be capable of providing for 100 much faster freighters to cross it?
For one thing, because the RAF were conserving their forces in case of an invasion. That makes attacking empty barges in full force along the French coast foolhardy. Much better to attack them on your own side of the channel, when they are loaded down with troops and supplies. After all, empty barges are rather cheap, and not a worthwhile target for large scale daylight raids on the enemy coast, with all the losses that implies.
The barges were not, for the most part, passing "through" the channel, they were approaching the channel hugging the French coast, thus hard to detect and never far from a port they could put into if attacked.