Dear Sri Gummuluru Murthy,
When you state that according to Shankara ' there is no other
source of knowledge for revealing the Non-dual Atman other than the
Upanishads' you have accurately portrayed Shankaras' position. However
when you put forth the view that " ultimately for Atmavidya, Upanishads
cannot be treated as requisite either. It is Mannana, Nidhidhyasana
which leads to Atmavidya, not the Upanishads." it appears as though you
have gone against Shankaras position and adopted a viewpoint similar to
that of an Ancient school of Vedantins which held that Brahman is
subservient to an injunction of meditation.(Dhyana Niyoga Vadins) These
thinkers insisted upon meditiation as the primary teaching of the
Upanishads,and that no indepentant entity (Atman) in itself could be the
subject matter of the Upanishadic teaching, as it would serve no
purpose. Shankara portrays this school of Vedantins in his Sutra Bhashya
1-1-4,as follows; "If it were only a statemaent about a thing, which
formed no part of an injunction or duty,it would be altogether
ineffective, inasmuch a there would be nothing to be taken up or
rejected by the seeker of truth, very much like the statements such as
'the earth consists of seven islands','There goes the king' " Hence this
school maintained that mere Sravana or study and interpretation of the
Vedantic Texts would be of no avail,and that is why after enjoining
Sravana (listening), the Sruti enjoins Mannana and
Nididhyasanana(reflection and meditation). So this school concluded that
the Upanishads are a means of right knowledge of Brahman only insofar as
Brahman is the object on an injuction of meditation.

Shankara's tradition is is of course opposed to this view, for it is a
sustained effort to show how all Upanishadic texts purport to reveal
the Non-dual Atman, and that this Knowledge is not available to any
other means of knoweldge( pramana) other than the Upanishads.