Get the more recent version of S2+ directly from Simon @ Convoy (through his official Aliexpress store), which has the biscotti firmware, allowing you to program it to just have a single mode.
It’s available with different tints as well as different number of 7135 chips (3, 4, 6, 8).
The 7135x4 configuration should give you around 450 lm output and draw 1.4A, which means a 3500 mAh cell will run for roughly 2.5 hours on max. In reality, it’ll probably run longer due to voltage sag.
For walking around the house, even the 7135x3 version should be plenty bright.
With that said, I don’t think I’d give my parent anything that runs on 18650 li-ion cell. I’d give them something that they can replace batteries in easily if they need to, so that would be something that runs on AA cells most likely, like this: