I have no real information *why* Atari chose FAT, especially since the big-endian 68K had to convert all the little-endian numbers before they made sense.
Maybe it was just convenience, because most 3.5" floppies came pre-formatted for FAT12. According to some information GEMDOS (one of the other parts of TOS, besides BIOS and XBIOS) also had functions that were very similar to MS-DOS, but I don't have enough knowledge of DOS programming to confirm that.
As for the working ST file not being mountable on OSX: Did you rename it to ".img"?
If it still doesn't work then, OSX probably does the same lame "trick" that WinDOS does:
Just read *one* specific byte of the bootsector and according to that decide what kind of geometry the disk has. TOS instead used the information on number of tracks and sectors per track, which are also available in the bootsector. This is why you can format a floppy disk with 82 tracks and 10 (DD) or 20 (HD) sectory per track in the Atari, which simply isn't readable on WinDOS, despite the fact that the floppy controller is able of decoding it just fine.
I have no idea how Hatari handles disk images, so I'm not sure what might cause the crashes. But I guess "nomen est omen": Hatari means "caution" in Swahili