I’m running into the same problem, but I can update something, forget about writing CV (host control mode pin) after power up to change the mode on the fly. This pin is only read on power up, much too early for any firmware to modify the MP2672 mode.
And I’m also running into the same problem: In host control mode (i2c lines connected to my i2c bus and CV tied high), if I power on my controller and the MP2672 is not powered (USB VBUS supplying VIN power not plugged in), then my i2c bus is rendered unusable for any device on it. This seems a bad design choice! The only way I can get host control mode to work is to power my controller with USB power plugged in. Not useful for me.
I gave up on this for now; not worth spending another week trying to figure it out. The register information would be useful and it would be nice to control charging from my controller, but in lieu of some authoritative answers here about the i2c bus status when USB power is absent, I don’t want to spend more time on it.