Even if they joined the Navy instead of sailing as pirates, it wouldn't have played out that way.
Ace was almost nothing but angst over his relation to Gold Roger. If he'd come to the same conclusion about his place in the world in the Navy as he did on the execution stand, that's it, he's not following any plan because he no longer cares about anything Roger related (Garp pretty much shot that one in the ass day 1).
Luffy definitely wouldn't given two shits about any plan. He'd be Garp's clone and if something bad enough happened to prompt it, he'd probably quit like Dragon did before him. Then potentially end up as a vigilante pirate anyway (a la Romance Dawn V1/V2).