I admit I have not done an exhaustive search of the uses of the pentatonic scale in world music. Wikipedia, as always, as something to say, and as usual, some additional references would be nice.
I see a striking absence of of the peoples descended from Germanic tribes. Iceland is often taken as a living laboratory, given the slow rate of change in their language til at least the 19th century, and it's absent from the list. From what little traditional Icelandic music I've managed to acquire, it doesn't look pentatonic to me.
So where does this idea that "the pentatonic tuning is the genuine ancient pagan tuning of the lyre" come from? Upon what shred of evidence does this assertion stand?
Check it: Drømde mig en drøm i nat, oldest extant Viking/Icelandic music. Diatonic in A Dorian, folks.
Hey, if you like to tune your lyre pentatonicly, you go do that. I used to. It sounds awesome. It's versatile. It's easy. It has many things to recommend it to a performer.
Documentable authenticity is not one of those things.
Own your performance decisions. You honestly don't need to take cover under hand-waving assertions of authenticity. You just need to state what's know, how you're varying from what's known and why you did that. Any other historical performer worth her salt will understand that there's a balance to be struck between what's documentable and what's playable/presentable, and that we all find that balance individually. She might not agree with you, but she'll understand you.