Just some thoughts....
I understand you not wanting to mess with the gable shingles, but.....
...something like this vent cover might work great.
Course you'd need a 6" hole saw, which costs twice as much as the vent cover:hmm
I wouldn't worry about weather stripping and/or caulking, just get...
This vent?
You can add a 'damper...piece of cardboard(or something) tacked to bottom of rafters to direct air up higher....kinda like this:
Not sure that's frostbite...might be a chicken bite scab...but those big floppy combs are bound to get bit.
Looks pretty good to me.
Maybe add some louvered vents at gables.
Have you measured the humidity inside and outside coop?
They should be pretty much the same.
I would only be concerned with adding more vents if humidity inside is ~10+% more inside than outside.
Have you stood inside of coop...