I might consider predrilling 1/4" holes in the scab boards to keep them from splitting. 5/16" is a big screw.

I know that virtually no one trusts glue and always wants to glue and screw, but I had an old wooden boat builder with years and years of experience tell me one time:

"If glue won't hold it a screw damned sure won't hold it."

If anything a screw should be placed in there to pull the wood together and hold it until the glue dries. Think of it this way... Once the glue dries, the glue is providing shear resistance across the whole faying surface. Screws are only providing shear resistance across the shaft of the screw.

When a laminate traditional bow is made or a laminate wood beam is made, there are no screws... Only glue.

Glue is badass. Screws... Meh...

So either screws or glue, but you really don't need both. Like I said each one of those screws will provide about 400 pounds of shear resistance. So 4 of them on each side of the crack would be more than enough.

If I was fixing that, I would cut 2 pieces of 3/4" plywood the same depth of the rafter and make them long enough to extend 12" on each side of the crack, dampen both surfaces, then glued the chit out of both surfaces with Titebond III, clamp them on each side, let dry, take the clamps off and go have a beer.