I agree with the comments to just fill it with glue (I just use super glue) and it should be fine. And about those tool marks...I have an Osage selfbow with similar tool marks on the back that I never got totally smoothed out that has been shot thousands and thousands of times with no ill effect. I have a Hickory bow with a few lingering tool marks from removing the bark as well that has also survived the test of time, although that one is a lighter bow pulling about 35#. I definitely try and do a better job of smoothing out the tool marks with a scraper or something these days. But my experience has been that as long as those tool marks, which look more like indentations than something that actually cuts the grain, don't violate a growth ring, the back of the bow should still be strong. Of course, others may have different experiences, and both of these bows are made of woods with superior strength in tension, so this may not translate across all bow woods.
Anyway, good luck with this one.
Eric