Need for ether is a good sign of low compression-either by design or by a fault.
The diesel fuel
must be raised to ignition temperature by compression
ratio of at least 15-16:1
There are diesels out there that were designed for 16-17:1. I have one.
It has no glow plugs nor pre-heater. (Volvo Penta
2003-but there are others)
It will not start properly if there is any compression leakage.
I suspect low compression after warm up can be caused by no clearance in valve adjustment. When cold.pushrods,rockers & cam lobes shrink just enough to let valves close. However,if there is no valve clearance,as soon as metal warms it expands,holding valves open a bit. This would cause low compression,poor power,poor running & hard starting...& eventually burnt valves.
Adjusting valves is no big deal.
You don't have to go thru the normal "finding TDC" either.
Put a wrench on the crank pulley.
Roll engine slowly til a particular cyl. has both rockers fully loose.
Adjust the valve clearance for that cyl.
Repeat with other cyls -one at a time
Must always be done after ANY head removal
Youtube or Google-valve adj. most any engines same principle.
Best of luck/ Len