Well, you're right that there is no (legal) way out. Once you pass through that secret wall, you are required (a) to fight, and kill, Bela (which has no effect on his appearance in W8), and (b) to take the spaceship ending to Bane/T'rang beginning to CODS.The good news is that it is possible to edit your party's location. I can do it, although I might not be able to get to it today: I am working as an election judge, which will keep me busy from 6:30 a.m. (half an hour before the polls open, to set up), until some time after the polls close at 8:00 p.m. If someone else doesn't step up between now and then, send me your game.
Also, if all your party members have ninjutsu, you might try the fight. Just keep at it until everyone successfully hides, and then kill Bela with cloud spells.