The real cause of injector rattle?

[Ruan]

That all sounds very true and reasonable. I have always been under the impression that coolant temperature has a large effect on the actual cylinder wall temperatures. Which would then have an effect on fuel atomization etc rather than it condensing against a cold cylinder wall. But that could be total crap and tbh I would expect it would have to be a large difference in temperature to make a noticable difference.

Where and when is the fuel injected on a DI diesel...

Go take a look at the piston bowl design - that might give you a hint as to why the cylinder wall temperatures aren't of massive consequence, other than being at ~80-83*C.

You've just explained why IDI engines are theoretically a better technology than DI diesel, just the practicalities of losing the heat to the head in a very small chamber are a pain in the arse. Not to mention the fact that IDI can generate better swirl to thoroughly mix the air charge than a DI diesel ever could.