I have Rebecca Whitman’s book on varnishing and will look up your situation.
But from my memory thinning varnish is a no-no. I’ve done it in many ways w no problem but the last coat should be max thick. Probably be good to have max UV protection on the last coat too.
Personally I think the best thing to do re varnish is to apply several to many coats on bare and dry wood w progressively less thinner (turp) and I usually just use Linseed oil. I know Tung oil is better but I’m not convinced it’s much better. And for best penetration I use kerosene for thinner on the first few coats.