You have to make a distinction between the manager remuneration breaches and the incorrect accounting breaches, I think.
Yes, the club should have easily complied with the P7 and P8 rules, but I think the PL is saying that, in good faith, the monies paid to Mancini should have been recorded in the club's accounts as part of his club remuneration. So, in their view, the accounts were wrong.
I don't think they can prove that, but the club's defence won't just be looking at the P7 and P8 wording. They will have to show why the services were separate, fulfilled and perfectly normal from a personal and professional point of view. I am sure they can.
Anyway, as we keep saying, it's not material to the accounts and very probably time limited anyway.