Since we agree we can't Know what Mahler had in , well, in himself, then how is the accuracy of Bauer-Lechner vital?
Think about it in this way:
1) In his post above Dave H (immediately after questioning the meaningfulness of doing so in the case of Mahler) cites words by Stravinsky from an unattributed source describing the birth process of the "Sacre."
2) You cite an anecdote about Beethoven's reaction to a question concerning his Eroica.
3) But we shouldn't refer back to Mahler in the same way because we cannot know what he "ultimately and definitively" meant by what he said and did.
What's wrong with this logic?
Sounds to me like an equivalent of spreading hearsay from one corner of our mouths while suppressing a factbook with the other.
Bauer-Lechner is a source for precisely that type of words and descriptions, too, one moreover that has born scrutiny remarkably well (with dates, names, numbers, all that notated down often in great detail). It should be much more "vital" as a source for insights of the above kind than any undocumented, unchecked, unpublished, unverified, unattributed, unreferenced re- and paraphrasings of something we may once have heard someone tell someone else somewhere. B-L is pretty much the opposite of hearsay: a record that's highly accurate and notably careful to all that we know from double-checking it.
It's just a source of information for those willing to learn more of Mahler, his work and his world. Scholars use her to establish timelines and other facts not available from elsewhere (e.g. from Alma's unreliable diaries). That's all. Of course she doesn't tell us "the meaning" of Mahler's work; it's not musicology or philosophy, it's recollections, even if it contains reflections of both kind in which the author shows an unusual ability to understand and expound on (technical and philosophical) musical subjects (not least thanks to her own training as a musician).
I wouldn't use it to tell me what Mahler means; she's not even trying to do that. For what it matters that question, in the context of Western classical music more broadly, has been much better broached by others who were musicologists, sociologists, philosophers, cultural critics.
Remember that more knowledge is always good unless you're a charlatan who wants to become an authority owning the topic; then more knowledge is your enemy. Right?
Now I've think I've said it all (twice [thrice?] already)... But I'm still not sure what the issue is, really, underneath.
-PT