Has anyone of standing (eg a former Secretary of Sate or Minister, the
UN, someone like the Institute of Strategic Studies, an eminent
academic), written on the notion of a third war being started by
Germany, by invading a neighbouring state (eg what are the chances,
how would it come about, why would it happen or not happen, how would
other countries react etc)?

Almost all general German history books deal with this issue to some
extent. You might have a look at some original cold war documents. You
will find that Brezhnev, the long standing Soviet leader was always
fearful of such a move and dealt with that in great detail. That is as
high and paranoid as it can get. The probably best source is PHP
(Parallel History Project) by the Swiss. You will find it very quickly
in a search engine. However, only a few documents are in English, they
are mostly in German, Polish, Hungarian, Russian etc. You will also
have to 'extract' the message as most of it is in old ideological and
diplomatic vocabulary. Good luck.