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Question

This passage comes from Proust:

Moi je ne sais pas, mais si j’en juge par moi, cela m’ennuie déjà tant de voir les personnes que je connais, je crois que s’il fallait voir des gens que je ne connais pas, « même héroïques », je deviendrais folle. D’ailleurs, voyons, sauf lorsqu’il s’agit de vieux amis comme vous qu’on connaît sans cela, je ne sais pas si l’héroïsme serait d’un format très portatif dans le monde. Ça m’ennuie déjà souvent de donner des dîners, mais s’il fallait offrir le bras à Spartacus pour aller à table... Non vraiment, ce ne serait jamais à Vercingétorix que je ferais signe comme quatorzième. Je sens que je le réserverais pour les grandes soirées. Et comme je n’en donne pas... – Ah ! princesse, vous n’êtes pas Guermantes pour des prunes. Le possédez-vous assez, l’esprit des Guermantes !

There's a lot going on in this passage.

1.

s’il fallait offrir le bras à Spartacus pour aller à table

I'm guessing that that is not an idiom, (it was translated word for word in Kilmartin) and I think it just means 'make a sacrifice in order to have a seat at the table'.

2.

Ce ne serait jamais à Vercingétorix que je ferais signe comme quatorzième.

I really have no idea what 'quatorzième' means here. Is this the name of one of Ceasar's legions, I know that it was the 13th that he used to cross the Rubicon but maybe he was using the 14th inter alia (pun intended) in Gaul and they made a special sign. Even if that's right, I still don't know what it means.

3.

vous n’êtes pas Guermantes pour des prunes

That was translated as "you're not a G for nothing", so I guess that passage is clear.

Answer 1

I'm guessing that that is not an idiom, (it was translated word for word in Kilmartin) and I think it just means 'make a sacrifice in order to have a seat at the table'.

No, the idea is not that of making a sacrifice. It is nothing more than a hypothesis of an extreme situation in which it is considered that the guests are very famous ("Spartacus" becomes by metonymy "a famous personage"), and the conclusion is that if boredom is her lot when she is among normal people whom she knows, not knowing those famous people and having to contend with the stiffness that their great fame induces in such social gatherings would be even more boring.

2.

To understand that point it is necessary to understand a superstition that apparently was sometimes taken seriously in high French society in those days.

(TLFi) quatorzième En partic. [P. réf. au fait que certaines personnes évitent d'être treize à table] Faire le quatorzième à table (Ac.1935).,,Être invité au dernier moment pour empêcher qu'on ne soit treize`` (Ac. 1935). Un petit garçon, qu'on fait dîner à la table des grandes personnes comme quatorzième (Goncourt, Journal, 1875, p. 1052).La mort ou une grave maladie sont les seules excuses à ne pas venir [à un dîner], à condition qu'on eût fait prévenir à temps, pour l'invitation d'un quatorzième (Proust, Temps retr., 1922, p. 1040).

"Vercingétorix" is again metonymically used as "a famous man".

3.

vous n’êtes pas Guermantes pour des prunes

It has been properly translated; however, the phrase "pour des prunes" is from colloquial to popular (TLFi, C), and this deviation from the current register is missing in "for nothing". I find surprising to see that Proust puts this phrase in the mouths of characters in the high French bourgeoisie of his time.

_{Note : My previous mention of the vulgarity of the expression « pour des prunes », which I had to change to comply with certain opinions of (user jlliagre) was not at all made on account of a would be indecency or obscenity of the expression; I never made out any such defect in this expression even before I learned its origin, a discovery I made today. I still think that vulgarity includes other concepts than those.}

Answer 2

To add on the other answer, about point 1:

"Offrir le bras" simply means to offer one's arm for the other person to hold, like for a dance or a date. In that context, it implies giving a "warm" welcome to an esteemed guest, acting somewhat flirtatious, etc., whatever arm holding entails in the society the story takes place.

I think it's the same in English, that's why it was translated word for word.