Hey guys, thanks a lot for the responses, it was really helpful.
Unfortunately I don't have too much time spend on the car but slowly things are started happening.
I'm updating this thread with the solutions for anyone who reads this in the future.
Following both advices I spoke with the closest dealership (Ancaster Shepperton) about the missing locking wheel nut key, and surprisingly they didn't started making problems about the missing keycard, so I just drove to them where they opened the magic master key box and after 2 minutes they know which key needed to be ordered, it costed me 18 quid and problem sorted
About the broken front strut bar thread, after some investigating I figured out that I would have no access to the nut from below, because the base plate for the front strut bar is just an extra plate welded on the top of the chassis, so there is no physical access to the nut at all.
I had no other choice but grabbing my multi-tool and cutting off the head of the bolt, it worked, the strut bar is off now, but finding a complete fix is a different story for the future.
Following the tip KyleR gave me about the rocker cover I just ended at ordering the Torquen kit which looks the best solution available (meanwhile I found it in the pinned topics as well).
Thanks to ZMANALEX for the extra advice about the spark plug tube. He told me I should replace it as well, because if the spark plug tube is worn the new seal would not help and it would still leak oil.
It absoultely makes sense (it's the same logic as a motorcycle telescope + it's seal) and luckily the spark plug tube is very cheap (4 quid each from Nissan), so I already ordered 6 of it from Nissan for 24 quid in total. Even if mines are in perfect order, 24 quid is well worth for me to make sure the fix will be perfect and I didn't need to disassemble the whole intake again (which is quite painful).