Converting two-vent fireplace insert into one-vent: how much combustion air access does it need?
I think you have considered all the right aspects.  However, even in our mild climate (Pacific Northwest), we opted to run both 2 inch flex lines, but we had only 12 feet to go.  It is our primary heat source and during our coldest nights we really appreciate the lack of draw through the cracks and crannies.

There is no harm in creating a draw through the surround, but a 2 inch circle seems like it is an invitation for trouble:  what 3 year-old could not resist dropping a candy bar or an unfinished hot dog through there?

Maybe it could be a slit the width of the side instead.