Er, if all that was the case, why does Reuss record that the day before the 1939 Swiss GP the German press was specifically instructed not to indulge in any speculation about the European Championship? I've only found one German paper - Welt Blatt (Vienna) - which went against that. Even press coverage of Lang well after the event - in September and October - doesn't call him European Champion (but I've found the odd mention of it in 1940 and 1941 in Czech and Slovakian papers). Why did Lang call his 1943 autobiography Vom Rennmonteur zum Europameister? Caracciola's autobiography also dwells on his championships in some detail.
As for Caracciola having to think during the Swiss GP: he didn't. He had no chance of the title under the old system. His target in order to win the title under the maximum points system was simply to beat Lang - unless Müller won the race, which would have given the Auto Union driver the title under both systems. Once Müller completed 15 laps Lang couldn't win the title under the old system either. So one simple pre-arranged pit signal telling him that Müller had completed his 15th lap but was no danger otherwise and the gap to Lang would be all Rudi needed. Again, I'd point out that Lang wasn't particularly good on a wet track - he had been unable to pass Müller at Spa and had waved Caracciola and Seaman (both good drivers in the wet: Bremgarten 1938) through to chase him - so why didn't Caracciola start to press him until the last third of the race when the track was getting drier? But it could be that Lang actually went against team orders - photos of him after the race in Cimarosti's book suggest that he might just have had an almighty bollocking for not letting Caracciola past. He certainly doesn't look like a man who's just won a race!
Also, bear in mind regarding the 1939 German GP that at the time it wasn't considered as crucial a race as it looks in retrospect. The cancellation of the Italian GP had not yet been announced, so it was still apparently the third race of five. Only afterwards did it become the third of four.
There is a contrary argument, of course. Which is that people forgot about the EC and it has come to be regarded as trivial or less important simply because they didn't understand it. Chula makes a similar point in his biography of Seaman - published in 1941. It would be another 11 years before a similar competition emerged. And as I pointed out above, in 1938 most German - and Swiss - sources that reported the race record that Caracciola was European Champion but give no further details in their Monday September 12th editions.