There is no "government national defense floor". As far as I can tell, this originates from someone misreading the SPR drawdown laws and then a bunch of people parroting it. The ~250M "floor" (really 252.4M) only applies for SPR drawdowns that aren't required by a severe energy supply interruption or to meet US obligations under the international energy program. Deciding what qualifies as a "severe energy supply interruption" is left up to the president. Essentially there's nothing stopping the president from emptying the reserve entirely. See sections d and h here: https://uscode.house.gov/view.xhtml?req=(title%3A42+section%...
> Public analyses have said that the cavern integrity becomes threatened once levels fall below 300M.
This isn't how the SPR works. They withdraw oil from it by pumping in water (oil floats on water, so the oil goes to the top). This means that the caverns won't cave in or something if there's too little oil, since they'll have the same volume inside them. What does wear out the SPR is drawing down at all, since the water erodes the caverns. The problem is cumulative, not caused by levels going below a certain point. There's a good article that goes over how the SPR works and its current status here if you're interested: https://files.gao.gov/reports/GAO-26-106918/index.html