It wasn't that Bai Long didn't have a backup plan, but his backup plan was a sort of last measure which Bai Long didn't want to use unless he had no other choice due to how much damage it would do to his own body and cultivation.
Still, by causing a lot of damage to himself Bai Long was confident in forcing Meirong back and then escaping alongside Linxin and the other two women.
But doing so would drastically change all of his plans and force him to live a life on the run till he was strong enough to fight back against the behemoth known as the Yin Plundering Sect.
Originally this plan of Bai Long was to just buy for some time, but he didn't add in the possibility of the envoy who came possessing a Ring of Transmission due to which she wasn't going back to the Sect and then coming back after consulting their Sect Leader.
