"I tend to best like the idea that It transcends earthly species and partakes of something of the essence of both ash and yew (and maybe every other tree), and so which mundane wood we choose to symbolize it is more about calling out certain aspects, rather than getting it "right."

This is probably the best way to approach it. Like you said, it's a mythic idea in any case, not a matter of botany. Still, I do have a soft spot for the ash as the one distinctive wood of Germanic tradition, and its properties and appearance also go very well with a Golden Dawn approach as I understand it - which is one reason I also got an ash wand to use for the HGD when that time comes.

"JMG's response was that prayers shouldn't necessarily follow the rules of affirmations, because they are a form of communication with a fully sentient other being, rather than a way of hammering something into your own subconscious."

True, that's a very valid point. My thought process here was similar to yours: this prayer, both in the Heathen and Catholic context, is more a personal declaration of intent towards the Gods as a whole, so I thought it was fitting. I agree that it probably wouldn't be right with the other Bedes.

Again, appreciate the encouragement, and I'll let you know when I've adapted the whole thing and hopefully gotten a chance to try it in practice (which is of course where the real value lies).