I feel that Bishop Fellay should resign for several reasons.
1) Archbishop Lefevbre DID NOT WANT a bishop as Superior General of the SSPX in the first place, so as not to give ANY of the SSPX bishops some kind of supreme mantle of authority, as exhibited by Bishop Fellay right now. A priest or a layperson was supposed to be in that position, and Bishop Fellay is entirely AWARE of this fact.
2) He admits to being "deceived" by BXVI. There are other priests who would not be so unwise to even dialogue with the Vatican until they've come back to the Faith entirely and without asking for any compromise except to be forgiven for being misled by the Devil himself.
3) Being in the position he's in, he is causing massive rifts among the faithful of the SSPX. Since the "conditions" were leaked, it's obvious that he is not doing what is in the best interests of the Society.
For these 3 reasons (and probably more if I thought of it a bit more), it's not hard to figure out that this is the best course of action for the Society and the rest of the faithful Catholics NOT part of the Society that he represents. Step down, with all due respect, Bishop Fellay, and let someone that is NOT a bishop be the Superior General. You will gain some of the respect you have lost, and continue with Archbishop Lefevbre's mission: THE SALVATION OF SOULS THROUGH THE ONE, HOLY, CATHOLIC, AND APOSTOLIC CHURCH, OUTSIDE OF WHICH THERE IS NO SALVATION.