I watched an old episode of Battlefield Detectives (about the Battle of the Bulge) and it stated that Hitler told the Japanese Ambassador an outline of this campaign several months prior to the start of the battle.
This got me to thinking. The entire world was at war and very few neutral countries, and very limited means to travel that distance.
How is it the Germans and the Japanese ambassadors actually traveled and arrived to their posts in the other country ?
They were already in place when the war began.
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Communications could take place by wire, which would go through neutral countries and enjoy diplomatic immunity (is there immunity if a diplomat is travelling to a co-combatant country via the high seas? Like, is it a violation of international law for the Royal Navy to sink a submarine which an ambassador to Spain from Japan was on?)
In theory, a Japanese diplomatic courier could travel to Germany along a route such as: Japan to Vladivostok, across the Trans-Siberian railway to far nortwest Russia, then to Sweden, then to Germany.
Note that even right up to Summer of 1945, Japanese diplomats routinely traveled to Moscow.
The big issue would be how to hop from Russia to Sweden. By air over Nazi held territory or Finland (which was at war with the USSR most of the time)? By sea out of Murmansk- but what nationality ship and to where? And probably not safe at all.
Maybe a southwest route would be better: To Turkey and then a lot of options to get to Germany from there.
A German diplomatic courier would have a much harder time. Travel across Asia would be impossible: not thru Russia, India, etc. That leaves pretty much submarine. Some Germans war materials were shipped to Japan via that route, why not a person? At least one German u-boat ended up in Japan Navy in 1943 and at least 4 were ended up in Japanese hands around the East Indies when Germany fell.
The Japanese ambassador to Germany, Hiroshi Ōshima, travelled from Japan to Germany in February 1941 (i. e. before Japan entered the war against the Allies) and stayed there until the end of the war [1]. Eugen Ott, the German ambassador to Japan from 1940 to 1942 stayed in Asia after he was recalled from his post [2]. Ott already served as German military attaché in Japan prior to taking up the post as ambassador [2]. His successor, Heinrich Georg Stahmer [3], had already been stationed in Asia (in China as well as in Japan) before he took up the post as German ambassador to Japan.
[1] Hiroshi Ōshima - Wikipedia
[2] Eugen Ott (Generalmajor) – Wikipedia
[3] Heinrich Georg Stahmer - Wikipedia
Also worth mentioning is the fate of the German submarine U-234:
The Vienna Convention on Diplomatic Relations, the international law source on the matter, provides that third countries have to accord immunity also to diplomats who transit their territory while travelling to their post, or from their post to their home country. Countries don’t have to issue a transit visa to such diplomats on transit, but if they do, they enjoy immunity on their journey even in third countries (Article 40). The Vienna Convention is also quite clear that diplomatic immunity also continues in case of armed conflict.
Of course the Vienna Convention dates to 1961, well after the war. But it largely codifies what was applicable even before as customary international law, so I would suppose that these were the rules applying in the 1940s.
I doubt that a country that would sink a surfaces sub with enemy survivors on deck would spare a submarine with an ambassador. 