To test the glow plugs with the multimeter, disconnect the plug wire/strap/bar that connects each of them together, and measure the resistance(ohms) of each one between the connector post and the engine block.
Yes, the compression is tested through the glow plug holes with an adapter. Harbor Freight has a reasonable price diesel compression test kit with the correct glow plug adapters. Be careful removing/reinstalling the glow plugs -they are not as robust as spark plugs and a broken off plug is a PITA.
After you have the gauge connected, pull the wire to the injector pump solenoid and crank -the gauge will hold at the highest compression reading. If it's low, remove the adapter out of the glow plug hole and squirt in about a tablespoon of engine oil through the glow plug hole, replace the adapter, and test again. If the compression goes up, it's the likely the rings are bad. If no change, it's likely a valve issue.
My vote with the info available is for bad rings being likely. These can usually be changed without splitting the tractor by pulling the head & oil pan.
You are getting smoke on cranking and you are getting spurts when the injector lines are cracked -this doesn't necessarily mean it's absolutely not a fuel issue but it's less likely.
