Just to start off away from the main topic: if Thecla, Dorcas and Agia are some combination of substitute wife, mother, sister, then presumably little Severian is a substitute son?
I skimmed forward a bit in Goethe's Faust part 2, which I've been finding rather heavier going than part 1 as it all seems rather inconsequential, even after reading the notes. Anyway, the following comment of the homunculus there seems to be pretty central:
Natuerlichem genuegt das Weltall kaum;
Was kuenstlich ist, verlangt geschloss'nen Raum.
Loosely translated to keep the verse form:
The world's too small for those of natural race;
The artificial needs a closed-in space.
`The universe is hardly big enough...' would be more accurate. This seems apposite for Severian before his trip `beyond the candles of night'. Further, the mandragora in Citadel dwells on its own non-living quality, and when Severian --- even when tempted as Thecla --- refuses to break the glass it seems to put natural life in the driving seat. Perhaps only Wolfe can be sinister and life-affirming at the same time.
I think there's a retrospective aspect, too. The Cumaean reference
seems spot on: the mandragora lives in a jar, has strange mental powers, and wants to die. Probably Wolfe knows the quotation from Eliot's use, like most of us. So it's a kind of postscript to Severian's journey across the wasteland of the old Urth. `Shall I at least set my lands in order?'
You could probably milk this link for all its worth. Phoenician sailors drowned by undines, Jolenta in the hyacinth garden, the Chapel Perilous of Master Ash, the Gyoll's tent is broken...
Hey, perhaps there's even a link to the seventies Dr Who story, `The Mask of Mandragora' ...
Shantih,
corncrake
or should that be cornflake?