Thank you all for your help and support.

I'm completely despaired, don't know what to do...
Someone on the French Miui Forum (an admin on the forum) told me I could safely downgrade to any 4.2.2 rom in order to re-flash Miui, as long as I wouldn't care about the knox counter... he did it himself

Also, you know the worst part of my story (which I didn't know yet when I initiated this post) : My microphone issue is still there, so I really didn't know what Samsung f***ed up with my phone. I'm sending it back to repair once again...

My last option could be to sell my phone and move on to another brand... except that I love the GN2 and its 5.5" screen size

In any case, this is very worrysome for the development scene... because any new phone (by new I mean newly repaired or sold) will end up with knox...

Android and Miui fan!
Phone: SGS III (rooted with Miui Rom + Boeffla Kernel) + Galaxy Note 2 with stock 4.3 (unrooted since stupid Samsung repair shop forced the upgrade with the latest 4.3 with stupid knox)
Nexus 7 (my 11-y-old daughter)
Old phones: HTC Hero (CM) > HTC Desire (Miui) > HTC Sensation (Miui)
Also owner of a jailbroken iPad2 ( oups! nobody's perfect!)