I think far too much has been dumped on what at its heart is a very simple idea.
A magical pilot-light.
And yes, Alchemy is absolutely a magical discipline - as much as Astrology relates to Astronomy. Alchemy is tied very much historically to the occult and the religious. The metals known at the time all aligned with the stars, the Zodiac played prominently, and looking for immortality and transformation not allowed by physics (at least without atomic process) all mark it as magic.
Its a good idea , but the restrictions on it simply don't hold well. Doing something as simple as requiring the heart of some rare creature would have sufficed.
This is an odd one - the selection of a name that is 1 character away from a real, famous guitarist is somewhat jarring, but okay. It seems oddly placed in time as well - comments about 'bards' and princesses while seeming to be an electric guitar .
Decently written, but I think the Metallica Guitarist dropped in the Middle Ages idea could have had a bit more work to make it fit a little better. An interesting idea to think on how they would manifest though.