it's nice if you can place your (top)speakers high enough so the sound can travel above people. That way everybody can hear it clear and the people who are near the speakers don't hurt there ears.
In a BBQ situation that's not a concern. People are milling about, and this is after all background music. My backyard is about 60x60 feet. When I want music I open the doors to my workshop and play it through the single David that resides there. It covers the yard and a lot more.
I don't have the Jack plans yet so let's say I might have theoretically gotten ahead of myself and ordered these parts, can I put them to use with a Jack 15?
You can't. The O15TB is a better choice for you anyway.