[Johnny .45]

Yeah, that sounds right. As far as I knew, inertia coupling is an effect of very high speeds and small wingspan. I don't think the Spitfire ever suffered anything quite that extreme...it was laterally unstable and hard to fly in a straight line, but inertia coupling is a potentially deadly effect. I'd think something more like "Dutch roll" would be the case. If it ever got fast enough to be susceptible to inertia coupling, it would have been in a very fast dive, not level flight.
So I can't say which is correct, and I'm curious to find out.
As for the cannons helping stability...I'm dubious. Maybe by adding a little inertia to dampen things a bit, but inertia has it's opposite, momentum.