Chris ,did you check that the gearbox morse control cable correctly engages the gearbox, ie has sufficient throw.
I too have had some gearbox dramas post a recently completed self done diesel heart lung transplant.
Cone gearbox TMC40.
Original gb ratio not correct so run in period consisted of inability to run up to correct revs, changed prop to seek a solution(easiest) but still had black bum, still not correct.
Changed gb ratio (always carry a spare gb ) and voila all was sorted, until at 200 hours gb failed to engage and drive in forward, removed and examined oil, metal in oil,bugger.
Changed out engine control morse cable etc, replaced gearbox as other one was toast and then paid very close attention to the engagement throw length.
Here is what I learnt, over my entire life I have had almost a obsessional aversion to paying anybody to do anything,so I tackle with a reasonably successful hit rate most of all life's tasks.
I now have a great respect for marine engineers and the contorted and complex environment they encounter daily.
My next boat will have the engine in the saloon and I will make the tea whilst the engineer goes about his business.
Part with the $$$ and pass the original problems ownership on to a professional.
Hah, wife has new car, I,m banned from lifting the bonnet.