Hello William! You bring up some very interesting points.

One point is:

Even though they [The Romances] are some of the earliest dated stories, they are not early enough to escape being changed, as sadly many if not all stories have been in Celtic culture.

Although they are early references to the Arthurian romances, they are believed to have survived hundreds of years in oral tradition, but weren't actually written until the 14th century. They must have been extreemly popular during this time, because we see a flood of Arthurian stories popping up around this time. Not only was this when the Mabinogion was put down on paper, but was also the time when Chretien de Troyes wrote the similar romances. Geoffrey of Monmouth actually wrote his Historia Regum Britanniae earlier than both tales, and shows that he got many of his stories from ealier Welsh traditions. He actually talks about how he contemplated writing in Welsh, but decided on latin to make his book more accessible.

I believe that the reason these stories sprung up about this time was due to the invasion of Britain, first by the Germanic Angles, Saxons, Jutes, et. al, then the Normans in 1066. The Celts wanted to preserve their own history and used Arthur as a national hero. Many of the Celtic tales were of earlier times when the Celts ruled the entire island of Britain, where now they were pushed into the fringes, such as Wales and Cornwall.

You also bring forth:

Do you take the Tales of Arthur as truelly Celtic? Even the early versions?

I take them as being Celtic mostly because of the motifs found in the stories. The Celtic use of numbers and colors are present in the Mabinogion and is very similar to that of other Celtic tales. The Celts were extreemly concerned with details in their stories, and the sign of a good story-teller was how many different details he could remember. This was very carefully monitored as well, as many already had heard the stories before and could tell if any details had been changed.

Finally, last point:

some of the worst ways to ruin the stories are translating them

I agree, and this is shown in the title of The Mabinogion. Lady Guest used mabynnogyon which was a scribal error in the earlier text. That just goes to show how scribal errors, as well as translation errors can change a text.

This is looking to be an interesting discussion. Thank you for bringing this up.