Empty barrel headspace

Looking for some experienced opinion here. Currently brewing an imperial stout with an estimated final abv of 9.5-10%. Our final (tertiary) stage is going to be in a bourbon barrel we secured from a local distillery that my buddy who works there hooked us up with. Problem is that we brewed 10 gallons in anticipation of a 10gallon barrel, but we had to get a 15 instead because all the 10s were sold to a tequila maker. That's a lot of empty head space area in that barrel. We were thinking of adding some priming sugar to the beer as we barrel it to put the yeast back into action producing some more CO2 and filling the head space to keep any unnecessary bacteria from building as has been suggested can happen in barrels with empty space. We will have an airlock in the bung hole as well to keep the pressure from turning negative once the yeast is finished with the extra sugar. Has anyone done this before? Will it work to keep the unwanted bacteria at bay?

Somewhere on this forum I read about a barrel being partially empty so the staves on one side dried out (it was on its side). The beer ended up getting infected which they believed was from junk getting through the gaps between the dry staves.

Brew another 5 gallons. Barrels are not air tight, they do breathe for lack of a better term.