Valholl was "wrongly transliterated into English in its genitive plural form as Valhalla". Has anyone else heard of this?
This seems to be true. I have also found this information on the web. The original way of writing seems to be "Valhöll".
In modern German it is called "Walhall", but "Walhalla" is also used due to Wagner's operas.
The term clearly consists of "Val-, Wal-" (the slain, also in "Walstatt" (battlefield), "Walvater" (father of battle - Odin) etc.) and "Hall" which is simply the hall.
To the second question, I think that the Celts and the Germanic tribes were, depending on the time and area, contemporaries.They traded and they married and it would be a surprise if they didn't also exchange ideas, even religious ideas. Iceland, for example, was settled from Norway as well as from Ireland. Dublin, the capital of Celtic Ireland, is a Viking foundation. In the ancient times, there were no walls between tribes and peoples and trade was more important than we would think today.
For example, Celtic nobility graves in Southern Germany contain wine vessels that were brought from Greece as early as 500 BCE over a distance of roughly 2000 km/ ca. 1300 mls. And the later insular Celts as well as the Germanic peoples, both had seagoing ships which made long range trading even easier.
Best wishes, and enjoy your book!