I like the idea. Probably could have given more context earlier on in the story.
Spiritual warfare isn't really all that common in stories and even then they are usually pretty obvious or have more tells to them.
Edit:
And again the worst thing that can happen is the MC becoming Better...
You don't have to dive for them. There are enough of them that wash ashore. There were communities who made a living solely out of washed up kelp harvesting.
So, is the fabled Roman Concrete now a thing? The one that gets stronger over time in seawater. Or is that the Summerstone I vaguely remember reading?
Edit:
What about using nets to catch the Ironborn boats?
Just the sap then. Soak or stain cloth and other woods in it to make it stronger, not rot and be immune to mundane fire and frost while resisting magical ones.
Would the ash work as Magic Fertilizer?
The sap as Paint, Stains and Dyes? Will it make wood as strong as steel, impervious to fire and cold? Fabrics that cannot be cut, burnt or torn? Will ships now be free from rot and barnacles?