Jolla's Sailfish OS is used as a daily driver by a small niche in EU, probably tens of thousands of users taking into consideration download and update traffic.

It's definitely usable as a daily driver. There are some rough edges if you only want to use native applications and want to avoid Android applications, which go through an emulation layer. But it's still pretty nice! Things like offline mapping work really well.

You can, by using the libhybris hardware abstraction layer I've mentioned. It is relatively simple, though still requires understanding of how low level stuff works. The main problems there are graphics and modem, both of which typically are not open drivers - and with that you then pull in half of your typical android system into your regular linux distribution just to get the hardware working.

If you don't care about the modem and graphics acceleration (or, in same cases, graphics output at all) you should be able to get it booting with the available kernel relatively easily.

Camera also tends to be not tends to be not that much of an issue - typically there's enough in the available kernel that you can get a gstreamer pipeline running, and then just have to figure out how to make it not look completely shit.

And after all that you still end up having a device with the baseband sitting on your SoC.