That's great, but if the end result is avoiding combat, Thunderspire Labyrinth wants none of it. The next encounter, entitled "Guard Room", states:
If the adventurers enter through the double door, regardless of how they gained entry, the goblins recognise them as foes and attack.That's an imperative. There will be a fight. And if the players avoid the main doors and creep in through the balcony?
If the PCs approach from the balcony and avoid being noticed, they gain a surprise round.That's a surprise round of combat. The initial flirtation that Thunderspire had with non-combat conflict resolution ends here, and doesn't resume for quite some time. From this point in, the players will be wading to victory over the corpses of their enemies whether they like it or not.
That's not all bad. Combat is what 4th Edition does best. And if we remember back, one of the possible quest hooks required players to eliminate the Bloodreavers, which was going to be tricky to achieve without a bit of hack-and-slash.
The real problem is that the players are offered a choice at all. Giving them three options for entering the chamber when they are all just going to result in combat will make players feel railroaded. If there's not going to be a choice, you're better off being honest and leaving a direct confrontation as the only available route right from the beginning.