Why is this a "major" problem ?
Surely an inspection by a QUALIFIED engineer needs to assess the "problem".
On the Isuzu marine engines built by HMI, if you turn the key off the engine keeps running, because the STOP button no longer has power. The stop solenoid is energised to run, so needs power to STOP. Procedure for stopping is to press the clearly marked STOP button, and onece the engine has stopped, turn off the key. This is all in the handbook.
The only exception is if you have an ex Black Prince boat, which you do not have. These had a module fitted behind the panel which allows stopping via the keyswitch. This module is retrofittable, but from past experience, HMI will not know what you need !! Maybe best to try Engines Plus, run by the blokes who ran HMI.
As far as the oiling is concerned, has the engine been filled with the correct spec of oil....API CC ?
If you've refilled at the first service with higher spec oil and done no hard running to bed the engine down, this may be the problem. I would suggest you give the engine a good thrashing....preferably on a river to avoid wash and bank erosion. It may smoke like hell to begin with but may well clear. This will burn oil out of the exhaust, but bedding the engine in is probably the problem.
Hope this is of help !!