The AIR file entry for the Fox Four Sabre doesn't match the MDL file name. (The only one I checked.) We never had a problem with it.
I downloaded your Fox Four Sabre, with no other F-86E, and it loaded. I added another F-86E, strictly named so in its files, although the sound and panel files are different. When I restarted CFS2, the new F-86E was listed - and loaded - but your KWCFS2_F86E had disappeared (no changes were made to its files other than sim=F-86E).
Changing the F-86E's sim= to F-86E Sabre and reapplying that name to the KWCFS2_F86E's sim= somewhat reversed the action: the KWCFS2_F86E had returned but was listed as the F-86E in the roster.
I think you're missing the point H. This is the Aircraft.cfg entry for the Fox Four F-86E :-
[fltsim.0]
title=
KW F-86Esim=
F-86E Sabre model=
panel=
sound=
texture=
checklists=
KB_checklists=F-86E SABRE_check
As I said earlier, the sim= entry points to the AIR file in use. The AIR file of the Fox Four F-86E is named
F-86E SABRE.air. If you change the sim= entry without changing the file name of the corresponding AIR file the aircraft will not be listed on the selection menu. The
title= entry defines how it will be listed on the Free Flight menu.
Note: Two other F-86E Sabres are included with the Fox Four campaigns, a South African Air Force variant & AI Only version. All have the same named AIR files, sim= & AIR file 101 entries. I repeat, this has never caused a conflict or other problems.