Excellent analysis. The issue regarding the Holy Grail being in England is it originally was a whole other magical cup. But, the storytellers changed it to the cup related to Jesus when England became Christian. The fact it would have been highly unlikely for such a holy relic to get out of Jerusalem never seemed to occur to anyone at the time.
And let's not forget all the stuff about the Fisher King and Grail.
Arthurian Legends are really all just one big clusterf**k of tales cobbled together from different storytellers each trying to make the tale relevant for their time. The elements pushing courtly love to the extreme are by far the weakest parts of the tale and should be taken with a large boulder of salt.
I'm certain at their core there was a "King Arthur" with a steel sword* and an intelligent advisor who attempted to consolidate all the local warlords in order to repel Roman invaders. He was successful for a time but ultimately ended up dying in battle. Anything beyond that is up in the air.
*Steel would have been seen as magical at the time. (500-ish A.D.?) Thus, why the Arthurian legends made such a big deal about it.