Nothing much of a preventative measure can be done anyways. Adding a little water wetter to the coolant may or may not help. I have no opinion on that.

Another thing I have observed. The head casting hardness of the aluminium seems to vary. This has been reported by one member testing various heads at one time from these engines. I cannot comment otherwise either on that.

That said one thing that is never mentioned but should be thought about. A running engine has or develops local hot spots. Before turning the engine off it does not hurt in my opinion to let it idle for a few minutes to equilise the temperature and reduce it slower than otherwise in the hot spot areas. if present.

Also this allows the oil feed to remain on the turbo bearings until the turbo spools down. There will be less potential coking of the oil. Instead sucking heat away from the bearings and bearing seals. As the turbo winds down.

I think if I owned one of these engines I would make a small cool down period a general practice if the trip length was long enough that the engine had really gotten warmed up.

Another factor to consider is that the block being cast iron and the head aluminium may enable a more controlled even contraction of the overall head with a small idle run before shutting down the engine.

Where the cracks appear on these heads is not really random. The location of them might be consistant with too rapid expansion and contraction in relation to other metal in their proximity. Especially if they are a hot spot.

All that would result in my opinion by running these engines a little hotter than they should be is perhaps more stress in that area between the valves during the cool down period especially.

These ports are not far apart either. So a consideration has to be the influx of cool air into the intake and really hot air leaving through the exhaust port. But this is just an item that may add or subtract from things.

If a bubble in the coolant were to form or develop from that overheated area with fast shutdowns or hotter engines at shutdowns . That could do it easily in my opinion. The stresses then could go way beyond what they should or otherwise be.

How this engine has been used may contribute in some way as well. Theretically is a lot of short trips over the years versus a mix of short and long etc. The aluminium in that area just might eventually fatigue out.

Sorry for the long response. Especially since there is no way to really prove this easily. Yet it still will not do any harm, Even if what I think is totally wrong to let the engine idle for a minute or two before shutting it down in my opinion.