If the plan after the first Test really was to just take Johnson out, have him work on his action and his rhythm for a couple of weeks and then bring him back for the third Test no matter what, then I think you do have to give the selectors some credit for it. I think it just sort of worked out that way though. Johnson didn't know he was playing for sure until the morning of the game (so he says anyway) and if Australia had won by an innings in Adelaide and Bollinger had taken ten wickets, I highly doubt Johnson would've played the third.
Equally, I don't believe for a second that "Johnson was always going to be rested at Adelaide." There's no way he was sitting out if he scored a hundred and took five in each innings at Brisbane, for example.
I think it's fairly apparent that the selectors panicked when he didn't bowl well in Brisbane, then panicked again after his replacements didn't fare any better in Adelaide, brought him back for Perth and put the best and biggest spin on it possible to hide the fact that they really had no idea and were changing their minds on a daily basis.