Heliduck, TCF
I dont know how much wisdom I can provide but I'll give you my best effort. As for the situation you described I would not be overly concerned with extending the ground idle time due to a slightly higher oil temp. The reason I say this is, we are avoiding the long ground run times not for oil temps but bearing temps. Oil temp is measured, on the inlet to the engine so if we squirt a hot bearing with oil that is a few degrees warmer to begin with the cooling effect will surey be less but I don't know if it would be enough to make a significant difference.
The bearing cavity is cooled through oil as well as through an airflow that comes from outside the engine just behind the module 3 near the T4 probes and is drawn through by venturi effect. When the engine is at slow speeds like at ground idle that venturi effect is reduced and cooling effect is also which can lead to higher bearing temps. Once the engine is shut down oil flow stops and the residual oil is left to cook away on the bearing. Even if the oil temp that is going into the engine is nice and cool.
It would be interesting to take a look at the rear bearing in your engine after a period of time to see if it has any coking accumulation if this is a flight profile that you use routinely throughout the life of the engine. My personal opinion would be that unless you are getting carbon build up in the filter, start failing oil flow checks, or find significant carbon build up in the rear bearing oil tubes I wouldn't be too excited. Nothing is for certain as there are lots of variables but thats my initial take on it from what you explained.
As for letting the fan run after shut down you are correct that it would only be cooling the oil in the cooler which, since oil flow through the engine is stopped, will never reach that hot bearing.
Wisdom or BS...you get to decide
Max