Very interesting, yes, and indeed supportive of the Norrington point of view with regard to vibrato if that's what one is looking for. What I was meaning to say about Norrington's tempi (bearing in mind that his Fourth is the only one I've heard so far) was that my problem with them is that they seem chosen to privilege an "expressivity of the moment" over and above the cumulative expressive character one achieves by taking more care of the interrelationships between tempi and structural elements; in other words the music ends up seeming patchwork-like (in my opinion Boulez goes too far in the other direction and his tempi are too subtly graded). However, as you say, the RN approach can give rise to startlingly poetic moments.

Who knows?

However, taking your points one by one:
(1) I find this a little strange - RN's interpretations are surely nothing if not logical, in so far as I imagine he could quote you some chapter and verse or other on absolutely every interpretative decision he makes - although you might not agree with them, or indeed accept his reasons for cherry-picking the "evidence" in the way he does, but he has certainly looked at the situation and made logical inferences from it.
(2) "Poor" in terms of orchestral balance might just, once again, mean "unorthodox", might it not? Clearly he has instructed the orchestra in terms of dynamics and likes the sound that he hears, otherwise he'd do it some other way.
(3) & (4) You're right, intonation and ensemble can be rough in RN's performances, though the same could be said for Furtwängler's, which WF aficionados dismiss as irrelevant to the heart of what he's doing. I tend not to agree with them but I wouldn't call Furtwängler incompetent.
(5) As I say, I personally don't agree with his tempi either, but I find it a bit odd that you seem to be judging a musical interpretation according to whether you "agree" with it or not! Aren't you also interested in interpretations which for one reason or another cause you to think about the music in a different way, from a different angle so to speak? That's what interests me about RN: I find his view of Mahler causing me to question what I take for granted in the music, perhaps especially when it rubs me up the wrong way.

Quite so, Tony. The clarinet in C doesn't sound quite like anything else.

There is nonetheless an interesting comment from Mahler in the 5th symphony at the entrance of the C clarinet in the first movement (at least in the Eulenburg score): "die ganze Stelle wird von der D-Klarinette ausgeführt". It's doubly interesting for me since it's phrased in a way ('the whole passage will be played on the D clarinet') which could mean a request to do so or just an acknowledgement that that would happen anyway! It's also interesting in that he specifies its replacement not by a lower clarinet (as one might normally do with a C part, playing it on the Bb) but by a higher one.

It's also rather unhelpful since at least in my experience D clarinets are even rarer than C clarinets. 

Mozart writes for clarinet in B natural in Così fan tutte as any fule kno. But in his day it wasn't normal to change the entire clarinet, just the two middle joints. I did bid on an ebay set of four German instruments not so long ago which included an instrument apparently in Db...

A pair with D and Eb was indeed so unusual back then. Indeed a colleague of mine has such a pair from Leblanc. He also used to have a pair of full-Boehms in Bb and A.

Oops. I had indeed typed not so unusual but inadvertently backed my indeed over it. 

I blame Yvonne Loriod and her not-yet-husband in equal measure.

I have this on order

http://www.mahlersheavenlyretreats.com/index.html

My learned friend has the advantage where it comes to the finer points of grammatical construction, as well he knows, and I wouldn't presume to have a handle on when Germans elide the second 'werden' in such cases, as they often do in practice even if properly they shouldn't. On the other hand '...wird von der D-Klarinette ausgeführt werden' (the correct future passive form, I think?) I would, however wrongly, have translated as '...is going to be played on the D clarinet'.

All the same Mahler's formulation is not the one he uses when directing that a passage be played on another instrument or even when giving the option: in the 7th symphony he has for example 'eventuell auf Alt-Posaune zu blasen'; in the 5th he also has a rubato direction as '...ist stets etwas flüssig-nachlässig auszuführen'. I did try to find a version which reflected this difference while not too drastically misrepresenting the grammar. Clearly I failed dismally.

Naja. Et hätt doch immer jotjejange.

Fraid not. You've lost the passive voice there.

The full correct form would be 'Die ganze Stelle wird von der D-Klarinette ausgeführt werden', since 'werden/wird' is used in German both to form the passive voice (werden + past participle) and the future tense (werden + infinitive). Ollie is of course quite right about the likely elision of one of the two auxiliary 'werden' in the future passive, but the point I was making was simply that 'wird von der D-Klarinette ausgeführt' is of itself an entirely normal passive construction.

'Eventuell' is a nice word: it means 'if needs be', I suppose, in the phrase Ollie cites ('eventuell auf Alt-Posaune zu blasen'). Certainly not 'eventually'! In that sense it's as liable to confuse people as its French cousin (as in the title of Boulez's essay 'Eventuellement ...': 'Possibly ...').