Welcome to BLF, @robdejonge! You came to the right place! And don’t worry, besides the community being extremely receptive, there are no “too n00b” questions, all of us were n00bs once.
Also, congrats, you seem to have really done your homework, impressive list of requirements! You seem to have left just one thing out: what’s your budget?
But regardless of budget, I think I can offer a few insights:
Re: 1-6, from my direct experience I can recommend the Wurkkos FC13: it’s an awesome light and fits all of these like a glove. I EDC mine in my belt pouch, and it’s my second most-used light: Wurkkos FC13 3500lm Flashlight, Reverse Charging, RGB AUX Button Light / Anduril 2.0 / IP68
About your general case: “looking under an airplane seat” and “walking around snake-infested wilderness” are two very opposite extremes, it’s not easy to satisfy both simultaneously. The first case (indoors) needs what we call “flood” (ie, distributing light more or less regularly in a wide area near the user) and can additionally benefit from a warmer CCT (ie lower than the 5000K you indicated and which are more friendly to the eye indoors) and high CRI (90+, which gives you better color rendition). The second case(outdoors) is usually the opposite and usually benefits from that we call “throw” (ie, capacity to concentrate the light into a “hotspot” that can illuminate as far away as possible),
works best with colder CCTs (my FC13 has the SFT40 LED at 6500K and perfect for that) and isn’t as sensitive to lower CRIs (mine is ~70 and doesn’t bother me when used outside).
That’s why I have two main flashlights: the FC13 for outdoors, and a 4000K Wurkkos TS10 for indoors. The TS10 is just perfect and everyone should have at least one, but it’s a 14500 light and so falls outside your specs.
But that said, if you insist in having a single light (and having been there once, I think I understand you perfectly), I would still recommend the FC13 but with the XHP50B LED: it’s 5000K as you indicated and has more flood, so it’s a better balance for indoors/outdoors use.
Now about your other requirements:
Beam tint: 5000K seems to get the most votes?
What we call “tint” is also related to DUV besides CCT (and that’s another deep rabbit hole) but yes, for your use case I’d say a CCT of 5000K is the more balanced.
Beam angle: are there flashlights in this category that let me slide the reflector to go from flood to spot?
See my comments above re: “flood” and “throw”. The “slide reflector” flashlights are what we call “zoomies”, and should be avoided because they generally suck.
Lumens: videos I see online showing 2000-4000 lumen flood lights make me go “oh yes, like that!” … so … that?
Most important for high-intensity applications (which don’t seem to be your case) are sustained lumens and unfortunately no pocket flashlight is going to sustain anything like 2000-4000 lumens for more than a few seconds (not enough thermal mass). But yes, the XHP50B FC13 is rated at 3500 lumens and according to 3rd party tests, starts at 3748 lumens and sustain above 3500 for over 30s and above 2000 for over a minute, and then stays in the range of 600-900 lumens for about one hour and a half. And 600 lumens is still a lot of light and more than enough for most purposes.
Make sure it cant roll endlessly.
The FC13 clip takes care of that.
Make sure it can stand on its tail?
Yep, the FC13 is totally tail-standing
Diffusion cap available for my ‘hotel bathroom’ scenario?
Available as an option: Wurkkos White Orange Diffuser for TS21/FC13 Soft Composite Materials Bright Indoor Reading Hiking Tent Lighting with Malleability, for less than 2 bucks you get both a white one and an orange one, the second is best for your ‘hotel bathroom’ scenario.
Submersible?
This is the realm of specialized diving lights, and it’s in direct contradiction with your “USB port” requirement unless you get one with a port hidden in the head threads area, but those are few and far between.
The FC13 is rated as IP68 meaning it should survive up to 2m underwater for short intervals, but that is contingent on its USB port cover being perfectly placed and in very good condition. My advice: don’t try that with yours, be it a FC13 or not, unless it’s a specialized diving light.
For normal usage (ie, splashes, heavy rain, etc) the FC13 and most lights are more than waterproof enough.
Fall protection / bezel / build quality.
The FC13 is rated to survive 1m falls and mine has survived harder ones with no issues. But don’t go around dropping your flashlight, if you’re in a fall-prone use case it’s strongly recommended to install and use the lanyard that comes with the light.
Is ‘replacing the LED’ a thing?
Needs specialized equipment and knowledge. Not something for beginners, but maybe further on.
Or would you just buy a new flashlight?
Even the pros that replace LEDs by the dozens don’t usually do it more than once on a single flashlight. Definitely buy a new one (that’s one way many of us ended up with a ton of flashlights).
Are there lights in this small form factor that offer really warm or even red light as an alternative to the normal light?
Yes, see the TS10 mentioned above for an example. And you can have lights with an alternate 2nd or even a 3rd “channel” with a different CCT LED and then “mix” them in variable proportion to reach basically any CCT, Hanklights are famous for that (but AFAIK none of them have USB ports and so fail one of your basic requirements).
Magnetic base?
Yes for the FC13, as a $2 option: Wurkkos Magnet Tailcap Work for TS11/FC13/WK03/HD15/FC11/HD20/WK15 Aluminum Material from Wurkkos Original Factory; I have one in mine and it works perfectly.
I hope the above helps, if you have more questions just let us know!
PS: don’t let the naysayers drive you away from the FC13 because it uses Anduril and is therefore “too complicated”. Anduril is just fantastic, and one click turns the light on and another turns the light off, and click and hold turns brightness up and another click and hold turns it down, and that’s all you need to know. Anduril can certainly do a lot of things but you definitely don’t need to use (or even be aware) of them all (I use maybe 10% and I’m definitely an enthusiast).
