I didn’t get it at the first reading as it’s a complicated sentence, but I’m pretty sure Parsing B is correct.

Here’s why : the qu of “qu’il m’eût suffi …” refers to un monde : “Un monde qu’il m’eût suffi qu’Allissa put craindre …“.

“Craindre” means to fear -, to be afraid of -, and cannot be used alone. With parsing A, it doesn’t refer to anything.

Il me plaisait (…) qu’il m’eût suffi qu’Alissa pût craindre pour m’apparaître haïssable aussitôt.

Craindre quoi ?

With the second parsing, both qui/qu refer to un monde.

A world [1B] that didn’t attract me much, and [2] that the sole fact that Alissa could fear it was enough to make it loathsome (“hatable”) to me.

The translation is a little clumsy but the meaning is there. I hope that’s clear !