Wow, thanks for all the great responses!
I think some facts are worth emphasizing.
1. We’re talking about an EDC, pocket flashlight. Not a primary backpacking flashlight which they’ll have in, well, backpack (likely a headlamp).
2. Any backpacker going on such trip ends up at cities, towns & villages most of the time. Most hikes are day hikes; most overnights are in huts, designated campsites etc. In other words, it’s not very big on being out in the woods at dark.
3. We’re talking about young women, medium built I’d say, 1.60m tall. Not gear junkies or EDC freaks 
So with this in mind:
AA vs AAA
AA will certainly make it easier to satisfy my requirements, but I’m afraid the added girth girth/weight will compromise the “carry-ability”.
It’s a small coin-pouch with: knife, flashlight, botane lighter (+ waterproof cover), whistle, and 2 plastic tubes for pain killers, water purification etc. I wish they carry it all the time, whether they’re going to party in city or on a hike in the woods. The comfort and discreetness factor is highly important.
The knife (Vic Walker- blade, saw, combo tool) is the biggest item at 84mm length and 14mm thick. I’m trying to keep everything else smaller.
A possible alternative would be to have an extra AAA battery in the pouch (wrapped) - adds just 12gr or so.
20 lumen vs 12/15
I considered both Thrunite Ti3 and Nitefox K3.
I compared my own lights at 3 different (advertised) output levels: 9 / 20 / 25 lumen. Based on that I think 12 to 20 lumen difference IS noticeable. I think hiking an unknown trail with 15 lumen is doable, but it would feel easier with 25 lumen. If you find yourself outside on an unplanned night-hike (lost your way/got hurt/had to turn around/whatever) you want this little bit of extra because:
- good chances you’re not at your best;
- you’re not prepared for a night outside so it’s important to move fast to somewhere safe.
I’ve researched the topic on this and other forums, and it seems that most people consider 25-30 lumen as the good-enough output for a night hike.
Regarding that Astrolux A01, I couldn’t find any reliable output/runtime information. Anyone has got the numbers for the currently available offering (I understand they changed the driver not so long ago)?
Thanks again for your thoughts and suggestions!
- Bamba