Do a compression test, outlined in the checking and adjusting section of the WSM.
The gauge, glow plug hole adapter, is available on Amazon for a reasonable price.
Could be a cracked head or broken ring.
That's a lot of hours, for an untouched Kubota diesel engine.
When this happens to me its caused by a clogged air filter.
Since I drive mostly on gravel road the fine dust will clog the filter.
Tapping the end of the filter on a hard surface is enough to dislodge the dust.
Used replacement engines are scarce for the RTVs. Rebuilt units are close to the price of new, when including the core and shipping costs.
That new engine should last a very long time, with proper maintenance.
Use this adapter for the glow plug port. The Kubota pn is 07909-31301
https://www.amazon.com/Lang-Diesel-Adapter-M8-1-00-LNG-TU-15-35/dp/B01MSMUR2R
Instructions to run the compression test can be found on page 1-S10 in the RTV900 WSM...
Seems that you addressed the obvious stuff.
Since the engine runs okay, with no fuel to cylinder 2, I suspect a cracked cylinder head or broken piston ring.
How many hours on the clock?
There have been reports of no radiator bubbles with a cracked head.
In my case the radiator was blocked...
I have a 2006 RTV with the same issue.
It's easier for me to just kill the engine to switch gears.
The pressure relief doesn't always work, while changing gears on a hill.
The first time I heard of this relay.
Here is a site that should answer your question.
Click on show models and then select your model to see the location diagram.
https://www.messicks.com/part/t0070-31410/relay
I cruised thorough your YT videos but did not see any mention of the engine work.
If you don't mind me asking...
What was done to resolve the cracked head and the new donor engine?
4. Is it a secret society that I'm not allowed to join?to get these parts counter people to explain their parts diagrams is crazy.
The local K dealer counter people have no clue.
Best to contact Messicks technical support, the best source for info IMO.
https://www.messicks.com/contact