If you use a "pressureless" method of diffusing CO2, like the bubble ramp, bubbling into a selfmade powerhead reactor, or bubbling into a fiter, NOTHING happens when CO2 ceases production. Apart from the fact that a non-producing CO2 mixture isn't really helpful, it will not empty your tank magically into a closed bottle. I promise.
If, on the other hand, you have the DIY connected to an external inline reactor, then DOOM-ON-YOU, all kind of nasty stuff can happen. But again, it is not so much related to the check valve, or dead yeast, as to the fragility of connections, leaky bottle caps, etc. A functioning check valve MIGHT UNDER SPECIFIC circumstances save your behind, so I wholeheartedly recommend it.
Oh, and if you connect the DIY bottle to the input side, you are asking for yeast-sugar-alcohol mixture in your tank, unless you use a very strong (not soda) bottle. But here again, a check valve will NOT prevent this.