Hello Jeromie,
So I managed to get my mm.cfg read from this path
C:\Users\<username>\AppData\Local\Google\Chrome\User Data\Default\Pepper Data\Shockwave Flash\System\mm.cfg
I dropped my tracePath directive as it seems to be ignored. Our PreloadSwf looks like this (I figured this should be the root of the sandbox folder)
PreloadSwf=\flashfirebug.swf
Security is indeed read from this path as JeanBonnot indicated
C:\Users\<username>\AppData\Local\Google\Chrome\User Data\Default\Pepper Data\Shockwave Flash\WritableRoot\#Security\FlashPlayerTrust\*.cfg
My flashfirebug.swf file is here
C:\Users\<username>\AppData\Local\Google\Chrome\User Data\Default\Pepper Data\Shockwave Flash\flashfirebug.swf
My preloadSwf uses the allComplete handler, which is not firing at all on Debug Flash Player PPAPI, I've filed a bug report here
Bug#3918993 - allcomplete handler is not being invoked in debugger version of PPAPI plugin