You can put the shellac right over the poly. It'll do the job.
Osage can have sapwood looking stuff mixed in the heartwood, usually in younger rings or those close to the bark. It also frequently happens around knots and forks or where two trees grow together. However, I am working a tree now that has super thick rings and I am finding whitewood throughout its almost perfect 19 years of growth. Go figure.
I'd get rid of that knot at midlimb. How wide is it there? It will casue problems with a bendy handle bow.
Also, I would not leave sapwood band-aids around knots. I have had them pop and lift a splinter in the heart.
Good luck. Keep posting.