Nick Pelling says that the "Manchu theory" does not fit the historical
record. Please note that all the historical dates in question refer to
the Manchu *script*, not the Manchu *language*, which probably existed
(modulo natural evolution) for over 1000 years -- as "Manchu",
"Jurchen", or any other name.
The "Manchu theory" is that some European visited the region sometime
before the 15th century, invented a brand new alphabetic script for
the local spoken language, and used that script to transcribe some
texts dictated by local people -- scholars, astrologers, doctors,
herbalists, whatever. (Other variants of this scenario are possible,
of course.)