Ive been keeping my eye on this thread as i did wonder if all the lines would come undone .
Its either a 10 or 11 mm as there is no imperial anything on Porsches and there is no 10.5 .. it should be an 11 which has corroded .
Your choices ..
Replace No.9 ..
Do a cut and join on No.9 .. i dont recomend that ..
Use mole grips on the corroded nut and either a spanner or more mole grips on the flexi pipe .. youll need to force it out of the bracket or at least far enough so it can be turned as the bracket locates the flexi pipe ... and whilst holding the metal pipe ( no.9 ) in place you will undo and twist off the flexi pipe .
Its important not to allow the metal pipe to twist .. it wants to be held ridgid as too much twist will fracture it .
Your kinda doing the job backwards basically .
You could cut the nut and re flair but if its steel pipes then i wish you the very best of luck there and if not it may well be too short to reach the pipe ... i think go down this route if my above suggestion doesnt work