Meanwhile I've done quite a few tests yesterday and I believe I have some interesting information to share :
- Test with an external coolant sensor. Results are clear : the ecu is not to blame as the discrepancies appear even if the measured temperature is far below the 85° point. I had to stop the engine quite quickly as it was getting vey hot without the proper temp information..
- The ticking was finally not coming from the N249, but from the nearby injectors.
Apologies for pointing to a dead-end, I should have listened more carefully. - In the same newbie-type-of-stupid-mistake spirit, I opened the black container underneath the N80 and it's definitely not a charcoal canister box but the secondary air pump whose cover was simply displaced.
Anyway I took the time to uninstall it and test it : I got a very strong blow of fresh air telling me it's working nicely. Now, this is were things are getting really interesting: I unmounted those large hoses connecting both PCV valves, the long one at the very top front of the engine (part 4 on the pic above ^^) and the small ones leading to the PCVs (parts 6 & 18). All of them were quite dirty (kind of thick burnt oil paste) that I quickly cleaned with some brake spray. Good news is that it had a good effect on the MAF discrepancies, not to the point of solving it but it slightly reduced the difference between L&R banks readings and most of all the car already felt quicker and less congestioned (for lack of a better word). Also I tried to let the engine run idle whilst hose #4 was unmounted and there was some smoke coming out the T-connector on the left (part #1).
Taking into account I haven't noticed any oil consumption, my uneducated guess is that there must be some serious dirt somewhere down below. Where are those pipes (#12 & 14) going?
Anyway, looks like I'm getting closer to the solution, and something tells me I'll have to unmount the manifold to see what's going on underneath. Of course the chase for air leaks is still on but I also expect to find some clogged pipes&hoses that need some attention.
and then depending on whether the system is under positive or negative pressure sending it to either upstream of the compressor or into the inlet manifold respectively. 
