A flat rate typically implies a set percentage of income, regardless of how much you make, with no breaks or reductions.

So, a 10% flat rate on someone who only makes $30,000 is $3000, while the guy who makes $5,000,000 pays $500,000.

Everything (including the passengers) inside the sub could have been fuel for combustion had there been time for the reaction to take place. If I remember correctly the interior of the sub could have temporarily been hotter than the surface of the sun during the implosion. Pretty sure just about everything burns at those temps. But the collapse and gas release from the hull happened so quickly I doubt there was time for anything to ignite.