I'm not convinced about the PL's FFP investigation. There isn't anything in the Der Spiegel stuff that would contravene those that I can see.
The only possible issue with the PL rules is that you're only allowed to increase wages by a certain amount if supported by increases in UEFA TV revenues, commercial revenue or matchday revenue.
However there's an alternative basis of calculation, which is that you're allowed to "baseline" wages to the value reported in the 2013 accounts and increase the level from there. In that financial year we paid off Mancini and his coaching staff, which we classed as wages instead of an exceptional item. That's because we paid their contractual 2-year severance agreement in full rather than coming to a separately negotiated arrangement with them. That meant our wage bill in 2013 was £233m and it would make complete sense if that was the baseline figure we used going forward.
Our employee costs in 2017/18 were £260m compared to something like £240m pro-rata over 12 months in 2017 (they were £264m over 13 months). We were allowed a basic increase of £26m from 2013 so our celiling before any other revenue uplifts were taken into account was £259m so there should be no issue as we've more than covered any additional requirements via CL media and matchday revenue alone, without taking commercial income into account. Our 2019 ceiling is £266m (2013 + £33m) but increases in UEFA/Matchday revenue alone is £47m so we could report wages up to £313m on that basis and we could also add in the average transfer profit over 3 years, which is something like £30m, if necessary plus any increase in commercial income not derived from Abu Dhabi.
In short, nothing to fear from the PL's rules therefore.