Re: Shingle cap vents
What you're proposing won't stop anything, particularly if the roof has eave or gable vents. The attic space needs to be vented.
My suspicion of what you have on your ceiling is "ship lap" siding, otherwise known as v-rustic. It is not tongue and groove, rather it has lapped edges. Either way, there is no seal created with this type of system and air is going to pass through any open crack or knot hole.
How your ceiling "should" have been done is drywalled and taped. This not only acts as a draft stop, but also helps create a fire barrier, and then over the drywall the wood is installed.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!