I am not an EE. Just a firmware/Linux kernel guy.

What a confusing datasheet. It seems to assume that we always will want an on/off control of some kind. Here, we don't.

So I would just connnect the 3.3V power supply directly to VDD_BAT, VDD_IO, and to VREG_ENA. The datasheet specifically says "don't do that", but only because they assume we might want to use a switch to control on/off without unplugging it completely. We don't.

And yes, leave VDD_CHG floating (not connected to anything).

If you are hacking the Betz board for this, then also cut the connection from the 2.5V regulator (or remove it).

EDIT: And I'm not 100% sure that we need to connect anything to VDD_BAT. Try it without that first. Page 10: VDD_BAT is required.

Cheers