The engine is just one "node" on the bus and in my experience isn't really a problem very often. Typical problem would beit something else on the bus, ie switch for hydraulic valve stepper motor.
There are basically 4 tractor companies and they are all world-wide, so yes EPA matters and has to be met, as well as everywhere else.
When your tractor is down and rain is coming you want quick solutions.
New tractors have a big lcd screen, they could have more online solutions and all documentation, etc. but service is where dealerships make their money and they want you to call them and pay 120$ an hour to have a kid sit in the cab with a laptop and read how to fix it in JD service advisor. A customer can't legally buy the tools the dealer has to fix there tractor. This is wrong and should be illegal.
The arguments about emissions tempering are valid, but there are existing laws for that, just because one way is easier to enforce doesn't make it right.