But then changing your values to not care about simulated torture won't prevent the extortion attempt either (since the aliens will think there's a small chance you haven't actually changed your values and it costs them nothing to try).

That 'costs them nothing' part makes a potentially big difference. That the aliens must pay to make their attempt is what gives your decision leverage. The war that you suggest is another way of ensuring that there is a cost. Even though you may actually lose the war and be exterminated.

(Obviously there are whole other scenarios where becoming a 'protectorate' and tithing rather than going to war constitutes a mutually beneficial cooperation. When their BATNA is just to wipe you out but it is slightly better for them to just let you pay them.)