I was surprised to learn that bibihang’s friend is running an XP-E2 from 2s using a Nanjg driver w/ Zener mod… apparently at 2-3A. It doesn’t work well (which is not surprising). See post #164 here. Clearly it’s a bad idea and not one which should be pursued!
It’s quite possible to run low current devices from a linear converter, even when output is massively lower than input. LDOs are linear and they are commonly used to supply from a few mA up to 100mA. 3.3v LDOs running on 10v+ input is normal. Building an entire buck section to run a 5mA or 20mA status LED wouldn’t be necessary for example.
ADV - your current understanding, expressed in post #4, seems generally correct. As Cereal_killer pointed out, running a ~3v emitter on 2s with a linear driver is bad. Otherwise everything sounds fine.
… now on to the things that haven’t been addressed yet: dirty power in buck circuits. Linear power supplies are known in all sectors for their clean power. Switching? Not so much. Switching always brings in some ripple or something. In the case of the cheap, hacked up, high current buck circuits we use the bad behavior can be extreme. Very tall voltage spikes in the output are possible, enough to quickly destroy an LED. So that’s another aspect to watch out for. There is some discussion of that behavior in this thread: ImA4Wheelr - Driver Mod: HX-1175b & HX-1175B1 (Pic Heavy)