I'm getting the feeling that HC's/the narrator's/the book's attitude towards humanism is changing. There is some doubt creeping in, both on the personal level with S himself (he is becoming a bit ridiculous - at least more and more divorced from reality) and on the ideological level.
HC challenges S: as mac points out, the two questions he asks S stump him and he has no answer. The challenge to the teacher, and the ultimate betrayal of the teacher are an essential ingredient in the evolving pedagogical relationship, in that it is only in this way that the student can free himself of the teacher, which is after all the goal of the teaching: independence for the student. the theme of the link between humanism and pedagogy again becomes important.
If the basis of humanism is transcendence in a very general sense, the teacher/mentor role in this process becomes essential: the guide. Nietzsche wrote: He who is a teacher from the very heart takes all things seriously with reference to his students - even himself. Regarding the erotic element of this relationship, I don't detect any sexual love from S towards HC, just a very generalised eroticism which is part of the teacher/student relationship. Now, be careful, don't get me wrong: what I mean by 'erotic' has nothing to do with sex (or at last only very generally as love of the other person). what I mean can best be understood by comparing 'eros' with 'agape', the other kind of love posited by classical humanism.
Eros = the love of the specific person, shown in friendship, mentor-mentee, family relationships, sexual partners (in its most basic -degraded? - form)
Agape = the love of the general; notions such as charity, mercy, altruism, love of humanity, of that which is human in everyone
As a humanist, S is motivated by Agape; as a teacher and friend and mentor, he is motivated by Eros.
Things are getting complicated in two ways:
first, the disturbing presence of Clavdia, who represents a competing influence on HC both on the personal level of sexual love, passion, and on the symbolic level as the EAST, an ideological threat to the WESTern humanism.
Second, S himself is swinging more to the Agape pole. his encyclopedeia, frankly, is a joke. Cataloging all the forms of suffering will not eliminate them. S is embodying a critique of humanism: it's possible to love humanity but hate people (perhaps 'hate' is too strong a word, but I think you get the point. We saw this also in BK, remember? It's a common trope of Russian lit, appearing also in Fathers and Sons by Turgenev.) The problem with humanism is that it can become too abstract, in the love of a perfectable humanity, the specific human is cast aside as not perfect enough. This is a problem inherent in all thought systems, though, not just humanism, and I certainly don't mean that humanism leads to the holocaust. I think Mann is introducing a critique of this depersonalising aspect of humanism.
It's interesting that the light f humanism begins to dim in the novel as HC comes under the growing influence of two forces: sexual love, and an awareness of death, in his visits to the moribund.