Can you use a planeswalker ability even if the planeswaker doesn't have enough loyalty?
Wizard's wording on the matter is thus:


  606.5. A loyalty ability with a negative loyalty cost can't be activated unless the permanent has at least that many loyalty counters on it


It doesn't specify how those counters get there. It could be through +X abilities, proliferate, or even cool things like Doubling Season. But they must exist on the planeswalker in order to pay the cost.