I stopped worrying about the silica content of wood ages ago. It's the clients call what wood I use on their projects I can try and guide them to a more sensible/cost effective product but at the end of the day once they get an idea in their head..........
Besides all that the silica content of proper timber makes little difference to modern tct blades (old school hss blades are a different story) and even something like ebony or iron wood that has a massive silica content is not as damaging as MDF or particle board.
I didn't know that about larch trees being used as a firebreak that's pretty interesting and also surprising. I've used green larch for a couple of jobs recently and when it's green there is so much resin in it that it feels damp and sticky to the touch so I would have thought that it would go up pretty aggressively.
Anyways was a partial success today cut one sill about 10mm long which threw things out slightly but overall I'm pretty happy with the dry fit. Wall plate sitting on top of the sill for a length comparison.