Carbon buildup is not a given. I idle my CRD all the time with either the A/C (usually) or the heat running, and using the rig as a power generator with a BIG inverter. The beauty of a diesel is that it naturally will reduce the amount of fuel entering the cylinders to match the demand. When idling, demand is VERY low, and the air / fuel ratio can reach levels of 200:1 easily.

I wouldn't worry about it much, a good "Italian Tuneup" is probably all it needs, if anything. Take it out and flog it for a bit, a couple of WOT runs on the highway entrance ramps is really all you need to do. These tractors love to work hard, but idling them doesn't bother them either. The turbo does NOT need to be doing anything in order to remain sealed (just heading off an old wives tale early) and even at idle, the turbo IS SPINNING quite fast. The vanes are closed, the exhaust is working the turbo and it is getting the full oil flow as normal.

BTW: Since we have both a pressurized intake (when there is boost) and a separate vacuum pump driven by the motor... Your use of seafoam into the vacuum lines probably never got near the actual intake and cylinders. I'm not entirely sure where the vacuum motor discharges, but you just washed out the inside of that motor with the seafoam. I wouldn't suggest doing that again, but you probably didn't hurt it this time.

You might want to change your oil because all that seafoam wound up in the crankcase.

The vacuum pump that allows your brake booster to work is driven by one of the balance
shafts which are way down in the crankcase.

Having seen the carbon build up first hand on VW TDI's with EGR (ALH and newer engines) the best cure is to keep if from happening

Some hear have claimed it was a VW thing and we didn't have to worry. Others found out they were wrong.
geordi is partly right. How their driven is part of the problem. On my last TDI I did a lot of crusing on the interstate in my job taking it easy at the 65 mph limit. That was when the most EGR was being used. Soot from the EGR and Oil vapors are what cause the build up.
It's better to stop it from happening by eliminating one or both of the causes. (pro-vent and GDE tune)

Some say to get the GDE tune but let the oil vapor remain and it will clean the build up. Sounds possible but who knows

not sure how to do more than you have. running seafoam in the fuel can't hurt it. I'm sure others will know more than me.

Running seafoam in a diesel engine CAN hurt it. Seafoam is alcohol based, therefore it can carry water past the water separator into the fuel system. The water will remain compressed and stable while pressurized in the rails. But once it gets to the injector tip and is free from the rail pressure it instantly vaporizes into steam. It does this explosively. Causing erosion at the nozzle outlets and the possibility of cracking tips.