I wouldn't worry about the headspace issue, because, as others have said, CO2 will displace it.
But if it doesn't seal, I definitely wouldn't leave beer in there more than a week or two. I wonder if it's actually supposed to seal and they didn't include a gasket or something with your order?
Oh don't give me none more of that Old Janx Spirit
No, don't you give me none more of that Old Janx Spirit
For my head will fly, my tongue will lie, my eyes will fry and I may die
Won't you pour me one more of that sinful Old Janx Spirit