One more thing on the hand wheel not disengaging; make sure all the old oil is gone from the shaft as well as the inside of the wheel and check for burrs or dents as you clean it. Any gummed up oil will act as glue and the wheel cannot free wheel. Lightly oil it with sewing machine oil. That won't gum up as regular oil does.
Bobbin problems. A Mdl 15 should use a standard bobbin case so it will stay put if it's in all the way and locked. Make sure it's clean and lightly oiled too.
Now, I don't know the correct part names so please bear with me.
On the oscillating part that is fixed to the shaft there is an arched section with a spring on it. This spring cushions the hook part I believe. On one of the machines we had the spring had somehow gotten bent and was interfering with the hook. I had to remove it, straighten it and reinstall it. This bent spring was affecting the tension and causing both bird nests and top thread breakage.
Just something else to check.
Joe