It's always interesting talking about the subjunctive in English, in the same sense archaeology is interesting. I always feel like we're examining plastic skeletal coccyx models and then talking about monkeys' tails: in English, you can get by thinking of the subjunctive as a nearly defunct relic of some ancient artifact of the language.
That's why I find you can't really talk about it intelligently (picture three dinosaur ribs, along with a plastic reconstruction of the rest of the supposed creature) -- without looking at its living relatives: how does the subjunctive work in French and German, or even Spanish, Portuguese and Icelandic?
All this just to justify my suspicion that "if I were you" may not be a subjunctive at all; or if it is, it's an entirely optional one based on how delicately you wish to avoid evoking something. After all, you can easily view "if I was you" as a past-hypothetical, related to "if I had a million dollars. This kind of past-hypothetical is regularly used to describe conditions (as opposed to the conditionals that usually follow them --"I would buy you a green dress"). So with conditionals, and the past-hypothetical verb that often accompanies them, there is no question at all of a subjunctive surfacing in the sentence.
To illustrate what I mean about whether your feelings play a role, you could say either of the following, of a woman you didn't think highly of: a) However pretty she may be, her popularity is no reason to elect her president of the student council. or b) She is obviously very beautiful, but that's no reason.... The distinction being that in a), you don't even want to acknowledge whether or not you agree she's beautiful, whereas in b), you don't mind doing so. It may be the same with if I was you: if you're comfortable you don't need to walk on eggshells with the person you're talking to, maybe you don't need a subjunctive to imagine taking their place.
Another point I've been thinking of sharing... in some thread I can't find, a learned teacher asserted that the English subjunctive is only audible or noticeable in the verb "to be:" were, be being the two examples given.
I'm pretty sure it's audible/noticeable/exists in the third person with all verbs: the usual -s suffix disappears when it rears its ancient head:
* It is crucial that he raise enough money;
* It is imperative that she find the time to attend; etc.
The last thing I was thinking of contributing was that dialogue frees the writer to be inconsistent, if he or she feels a character would say something a certain way. For example, when Bill Clinton spoke at Oxford, he would use more standard constructions than years later, when on TV in the States. Educated people can avoid sounding educated when they're playing the modesty card. Maybe the character was depicted at that point as not wanting to outshine the interlocutor with good grammar. (I don't know).
In any case, I enjoyed reading the Le Carré work you're talking about, but I find he's such an ordinary writer (as opposed to story-teller) that I usually forget 100% of his books within weeks. Is he good enough a bard to examine with such a fine toothed comb? I'm not sure he is.