Actually I keep lousing the measurements when in a hurry. It seemed like those were fast seconds, but I made up for it by going to 500ohms. 1.25S at 3.1 V, I think that will still work out, but I was looking at 47uF caps in more detail and man, those DC voltage vs capacitance curves for 10V and 6.3V caps (only ones available in 0805) are nasty, so it will need testing with a real CAP (but a v-chip can still help too). Also I wonder what our rules are. TA you're the driver arranger. What are you interested in trying here. I'm betting that adding capacitance to the traditional OTC slot helps almost every bit as much as adding it to the C2, but I'm not sure. Possibly matters how pin states are set (something I haven't understood or played with enough). It depends if we're thinking new boards or just keeping the ones you have. If keeping them, might as well use the spot. Then you can get 100uF anyway.
I think 5.0V LDO is a much better choice. 5.0V gives you much more off-time power. PLENTY in fact, but 3.3 is down on the low end. Of course we want it to work at 3.3 anyway, but's no need to cause issues for people with multi-cell lights and lousy caps. More importanly I'd like to keep the resistor divider simple, so two equal resistors for 2S, no divider for 1S, two to one for 3S etc. If you have a 3.3V ldo, you can't put 4.2V on the sense pin, it's way past spec. You can only go to Vcc +0.5 V. So that makes resistors more complicated and at the same time reduces the margin for error in them before hitting the cuttoff voltage. There's enough margin, but then you really have to get it all right. With 5.0, 2-S, just use any two equal resistors and you're set. 5.0 would also allow auto detect but that's not going to happen in a 2048 byte bistro anyway.
Otherwise, setting this up for multi S won't be a big deal. Forget tailcap lights for right now, I'm too lazy at the moment to think about them, but without that and with a 5.0V LDO, you just need to divide the voltage by two for two cells. If you want to skip a bleeder then you'll want the total R1+R2 to be low and within some window, but there's still a ton of flexibility there. No more of this 19.1 vs 20 vs 22 nonsense. That won't matter, so long as they're both the same and both in the right ballpark (I'd say two 250 ohms each is looking pretty good). You could instead have them higher(but still equal) and use a 500 ohm bleeder, but it's probably completely unnecessary.
As for the software, it's just one config option and getting the right calibration table in place, which will depend on the LDO voltage so best to choose one and stick with it.
At the moment I'm using no R2 resistor for 1S. That's probably fine and even R1 could be shorted, but to bring the voltage down slightly farther into spec a 10x R2/R1 would be ok too (so backwards from the past). It probably really doesn't matter though.