That's a tough one. Homosote.
Yup, that's the old style insulation which was used on the exterior siding when shake siding was installed. That probably means you will have the same issues later on if you used the same material. My recommendation would be to use Styrofoam closed cell insulation boards ...for next time if you did not already.
I would wait until you could do the entire roof at once ...some day.