I don't know exactly how that camera handles back-button AF-C with AF-M -- the X-T2 has a separate "Shutter AF" option for disengaging AF from the shutter release and my X-T10 doesn't -- so I can't help you there. I'd assume, however, that it's normal -- the camera's just waiting for the lens to finish focusing.
I personally don't recommend back-button AF-C for general use. If you really need AF-C, then you need it. But if you've got it set to AF-C just in case someday you want AF-C, be aware that if your shooting aperture is at f/9 or higher, the camera will not use phase-detect AF. If you're controlling the aperture, it's usually easy enough to avoid that issue. But if you're shooting P mode, in daylight the camera might select f/9 or higher, leaving you with unnecessarily slow contrast-detect AF.
Fujifilm doesn't publish most of this stuff. That's why there are so many third-party books, such as Rico Pfirstinger's that are advertised on this site. Pfirstinger, for example, recommends skipping AF-C for simple tracking AF when you're only going to take one shot. He recommends using AF-S and "mashing" the shutter button down in one stroke, skipping the half-press. That way, as soon as the lens has focused, the shutter is released. That should give AF results as accurate as AF-C.
And yes, in single-point AF, the camera will try CDAF if PDAF fails, regardless of which PDAF point you select. Unless you're in AF-C with CH drive, in which case the camera uses only PDAF at apertures of f/8 or below, and only CDAF at apertures of f/9 or above.