Re: What if ...
> Wasn't FAT based on one of the hundreds of CP/M formats.
No. There were not "hundreds of CP/M formats". There was one file system implemented on many different capabilities of hardware. For example drives may have been hard sectored or soft sectored, FM or MFM, single density, double density, quad density, 40 or 80 track. 8, 9 or 10 sectored, with various skew factors. These different types of hardware were chosen for various cost, performance and availability reasons. For example particular controllers were slower and required a longer time between sector reads and thus a particular skew factor and/or inter-sector gap.
Even with [MS-]DOS there were still 'dozens of formats'. The original diskettes on an IBM PC were 160Kb. Other companies used different controllers and drives and could not directly exchange data with different machines. PC-Alien could read and write many 'alien' CP/M formats _and_ many 'alien' MS-DOS formats.
It was only when all manufacturers started building IBM clones that they (mostly) used the same drives and controllers and became compatible with IBM PC diskettes.
But, no, FAT file system in the way it allocates and records file sectors is completely different from CP/M's. The only similarity is part of the directory entry so that the FCBs can be compatible for converted software.
> As DOS itself was a rip off of CPM/86.
No. It was a rip off of CP/M converted to 16bit 8086/8088. It was probably CP/M 1.3 because PC-DOS 1.0 had a bug in the FCB handling on a close that existed in that version of CP/M. Both Microsoft and SCP were OEMs for DRI CP/M (MS for their Z80 softcard) and had all the code that DRI gave to OEMs.