"would not likely be objective when conducting HIS OWN MUSIC...how could he be, if by objective we mean dispassionate and detached"
To me, "objective" would mean following the indications in the score as closely as possible. To me, "dispassionate" and "detached" are subjective observations on the listener's part. I'm willing to bet that there were moments when even Mahler could appear dispassionate and detached with his own music. But that's not really an important point because we don't have evidence either way; and, "d. & d." are simply observations on the listener's part anyway. I think it's interesting that on Mahler's own piano rolls, he played the final stanzas of the fourth movement of the 4th symphony much quicker than most people do. So did Bruno Walter; probably wanting to follow the wishes of the composer. Is that "objective" or "subjective"? I would call that objective, because it comes from the source. What Mahler was depicting was a boy's view of what it's like to sit around in heaven, and not some comfortable songfest to make some fat (or skinny) soprano look and sound cozy. The singer and harp player need to adapt to what the music is depicting, and by whom (and not the other way around!). From all accounts, this would be very much in line with Mahler's approach to opera. He was not interested in making beautiful sounding singers feel comfortable. Instead, the singers and orchestra were made to adapt to what it was that the text and drama were depicting. I'd say that that's pretty darn objective, assuming that one understands the text and drama really well. Certainly, this is why his Wagner was so universally praised, whether in Vienna or New York.
Yes, I agree that Klemperer was striving for "objectivity" in his earlier years. He was, generally speaking, a fast tempo conductor when it came to the standard Austro-German classics. My understanding was that both Stravinsky and Schoenberg had some reservations about him conducting their music; but that they both, by and large, approved of his efforts. I very much like his recording of the Stravinsky "Symphony in 3 Movements", even though that comes from his late Philharmonia period.
So, in many ways, I think that we're both agreeing on a number of things, but are simply looking at the usage of the word "objective" differently. I think that Klemperer's Mahler 7th is good at "texture" type issues (dynamics; balances; clarity of line; coloristic effects, etc.). But at those glacial tempi, it bloody well ought to be! I still think that the old Penguin Guides - which I loath - came up with a great description when they said it was like listening to "Elektra" in slow motion. My only contention is that - and if it were possible to prove, I'd bet great money on it! - is that Klemperer would have conducted it considerably faster in his earlier years. Certainly Mahler's own timings were considerably shorter, and Klemperer was there at the premiere in Prague. It doesn't really matter whether we call that subjective or objective, the differences in timings remain quite stark. I would bet a huge amount of money that Klemperer wouldn't have dared to try his glacial tempi in front of Mahler! But that's a moot point.