Reliability
I can see your frustration, because it was apparently running like a dream before the exaust repair work started. The siphon break is VERY important, because otherwise you can get water in your engine = VERY bad. Even spinning the engine too long with the water intake seacock open can be a problem. Have you been monitoring your oil color (black good, gray bad)? I'd probably check the exhaust elbow too, as that can cause stalling issues and clogs easily. Plus it won't cost you anything and should be a routine maintenace item anyway.
It is worth the effort to get the diesel going. A working diesel adds thousands in value to a boat. By the way, regarding reliability, you can't beat a diesel...we subsequently ran mine for an average of 12 hours a day for three days without a hitch. I wouldn't want to try that with an outboard. 300 hours is nothing on a diesel.
I signed on one of those $25/year diesel sites (boatdiesel.com)and got more and better information there, the internet and here than anywhere else. Were I you, I'd try to as much diagnosing as can myself and then hire the expensive diesel mechanic to do the one off stuff, like a compression check, as needed. It could be as simple as a weak governor spring, or a bad starter from my brief skimming of boatdiesel.com, but you should do some home work yourself and not take this as gospel. Start with the basics...fuel is good and air free...compression is good and spark is good (connections/battery).
I bought a bunch of parts I hopefully won't need, but it is always good to have spares and I know I will keep the boat for a long time. I think of it as an investment. Given the expense of a mechanic, it might be a reasonable investment to throw a starter in if it seems to be cranking slow. It may be that the new exhaust has added just enough resistance (back pressure) to expose a weak starter.
It is probably worth posting to see if someone can recommend a good mechanic too. I encourage you to post all the details you can and you will likely get better opinions than mine, as at this point my experience with diesels is VERY limited.