No matter what happens, the proper batter is always the one that follows the last batter, and if there is a proper appeal, the proper batter is the one called out, to be followed by the batter after him in the lineup. Thus, only the last two batters are ever relevant to the appeal.
The thing to remember is that it's better to think of this as MYTAB (missing your turn at bat.) The batter that misses his turn is always the one called out, and since he's out, the one following is now the proper batter.
In your case, B7 batted in place of B6. If a proper appeal had happened after the fly out (before a play or pitch to the next batter) then B6 (the proper batter) would have been called out, and B7 (the one that follows the proper batter) would have batted (again, in this case.)
Since they didn't do that, the first pitch to B6 legalized B7's at-bat...but since B7 was legalized, B8 should have been the proper batter, and B6 is now an improper batter, drawing a walk. If the appeal happened immediately, then B8 is declared out, all runners advancing on the walk return, and we resume with B9. If the appeal occurred during B8's at-bat, B6 is legalized, making B7 the proper batter, and he replaces B8 in the box.