Ok to start I do know a little bit about hybrids I know to only use attys with protruding 510.
↑this, OP, I assume means that you know that your biggest safety concern is creating a hard short with a pseudo-hybrid setup. You didn't state whether the gear your buying is authentic, so there's a question as to whether the +pin on the "Derringer" 510 protrudes far enough to avoid an explosion. Or whether that +contact will continue to sit proud of the surrounding 510 negative body with use/time. You also know that you can (over time) dimple the top of your battery, with this arrangement, and that can cause a hard short too (leading to thermal runaway. . .boom).
So, these are, by exponential degree, your biggest safety concerns with running this setup. And your statement (quoted) is, I assume, why this thread isn't focusing in on the real safety concerns here.
You are not going to run into safety issues with how deeply you discharge your battery. You would have to be dead-stupid-drunk to run your battery so far down as to create safety issues. There is a noticeable drop in vape quality around 3.5V. At 3V, you're almost not even vaping.
If, if, if you were to continue to "vape" (when it's no longer producing any steam) all the way down to 2.5V (manufacturer's specified discharge limit), you still wouldn't be instigating catastrophe. (you might want to buy some new batteries, sooner rather than later)
Over time, how deeply you discharge your batteries will have some effect on their performance and longevity. This, also, wasn't your question tho. (whether you'll need to drop another $8 in 7 months or 12 months for a new battery). The only scenario in which you might have a bit of a concern is if you repeatedly tax your batteries by running them down too far and then demand the highest degree of output from them. Meaning, don't discharge your batteries to 3V a bunch, and then decide to super-sub-ohm with those batteries to where your drawing most of the Continuous Discharge Rating.
So, the topic of *safe* discharge levels is moot, really. Both in the sense that your safety concerns lay elsewhere (pseudo-hybrid mod paired with "whatever" atty), and in that it takes comatose numbness to vape a battery down to where it's dangerous. So, don't worry about it. (worry about the other thing!!)
A NOTE: I'm posting this, not in reply to anything . . . it's almost out of context. I just realized that others will come along and read this thread, and we didn't cover the real safety issues here . . . not specifically anyway. Which could be misleading.