Well I know that you have had problems on jeremy. I was there for the one of the first sea trials, Helping to get the alarm system up and running.
Just a few things that I am seeing is
-Why was the boat signed for If there was such a deviation in the Top Speed , cruising speed and Engine Max RPM.
-We have exactly the same engines(550 @ 2100) and at full RPM/load we are only burning 100lt/h on each main, so possibly FT is giving an incorrect fuel consumpion reading (this can be calibrated) ,the most accurate one is from the cat ECUs wich you can read of the mains control screen)
-When buying a cheap boat which is the 2nd from a new chinese shipyard that you have not had a build captain or engineer on it would have been advisable to do extended cruising in the area before leaving beyond a range where the shipyard has no ability to repair.
-I feel that most prob your owner has been not fully advised by his previous engineer and captain as to recommendations on taking over a new boat and the problems that can arise and be avoided if prepared for them.
Glad to hear you are getting everything sorted. We to as most boats have a harmonic vibration at a certain RPM. it is very hard to get rid of it completely as it is as hull engines all come into harmonic vibration. let us know how you get on.