Measuring power usage over time? What kit is that?

If Espruino is in the middle of a function then it'll be using the full 'run' power, but if it's waiting on a high-res timer it'll be using the non-deep-sleep power (10mA?).

The current DS18B20 module actually cheats - it returns the last sensor reading and kicks off new one, so as a result the Pico won't actually be awake for very long at all.

I'm afraid there's no clock slowing implemented at the moment. You could probably do it with poke if you needed to (but there might be issues with USB dropping out if you're not careful). You could also compile your own version of Espruino quite easily I think that just ran at a lower clock speed.

I'm actually be quite careful with 18650s - they'd be normal Lithium Ion? I'm not sure what the self-discharge would be like over 18 months. You can get non-rechargeable Lithium cells that are rated for 10 years though, and they could be a good bet?