As to the 286 version of "Concurrent", and the IBM 4680 / 4690 base, that was what was eventually named FlexOS. It was initially named Concurrent DOS-286, and the early versions (1.3, 1.42) can now be found on bitsavers, together with an '386 version of 1.40 & 1.42. A 68k port of an earlier version (1.20) named CDOS-68k can be found on Gaby's site.
The '286 versions seem to include the hinted at "MS-DOS Front End", and the 68k version apparently contains a "CP/M-68k Front End".
However for all of these, I'd suggest they have no direct code connection with Concurrent CP/M-86, merely specifications and related concepts, together with similarities in some of the core structures. The FlexOS I used commercially was written in C, with bits of assembly support logic. I was still using FlexOS 2.32 in '97 until I changed employer.
At most, I'd suggest that the MS-DOS FE may be related to the CCP/M-86 PCMODE component, and similarly the CP/M-68k FE may well be the CP/M-68k C source adjusted and recompiled. However I'd also suggest that the majority of the rest of the system is new code.
I do recall that DREDIX came with FlexOS 2.x, and that there is C source for it in some of the older Concurrent DOS collections, but the OS kernel proper strikes me as almost (if not all) new C code. In FlexOS 2.x the DOS FE only existed in the 386 version, building upon the V86 mode.
It would certainly be interesting to find more details about the early proposed MP/M-286 which is mentioned in some magazine articles. Was that something which actually started using the CCP/M-86 code, or was it simply an early marketing name for the FlexOS development?