I thought things couldn't get any worse, but this afternoon a backfire smashed the crossover pipe into my radiator top tank making a crack about 3" long! :(

 

So you're saying if you turn the key on and wait a few seconds it backfires after a few seconds or when exactly is it backfiring. This is different then what others are experiencing. Soon as you turn the key the ignition should spark. If you have it set to fire the prime pulse 2 seconds after key turn it shouldn't have any fuel to ignite. So if your MS is firing the priming pulse after two seconds and then igniting it something isn't right because there should be no ignition events until you actually crank the ignition over.
Ignition hold can be used to delay ignition until you've the engine has seen a couple revolutions. I don't see that helping you though if you're not actually turning the car over.
My backfires were never that intense and they were always on out the exhaust it seems so I never lost any couplers or vacuum lines.