I think, that what you asking is heavily depending on the implementation of the specific camera. All modern mirrorless cameras that I know (and probably most somewhat modern cameras) do not cut the power the moment you throw the switch.

Some even do a sensor cleaning, close the shutter etc then - so whether the engineers decided to do a hard stop to operations or not, is simply their design decision.

I would guess that most cameras would try to write out the buffer before turning off that sub-system. This might exclude very simple, old or cheap cameras.

Your assumption that the buffer is written sequentially is correct. So if the camera turns off file operations in mid-write, in best case you have one single corrupted image, in worst case a corrupted SD card, as the filesystem also needs updates written to it. This could be the case if you disconnect the battery in mid-operation (if the camera does not have a capacitor to counter even that).

A word of warning: At least I have non-recoverably killed SD cards during experimentations with raspberry pie computers - but that is not comparing apples to apples, as there I actually can cut the power mid-operation.