"Ursäkta mig" is the same as 'excuse me', when you try to pass someone in a store or if you did a not so severe mistake. "Skulle du kunna" is incomplete without some action/verb. It's equivalent to 'can you [pass the salt]?', 'would you be kind to [help me with these bags?]' or if said in an upset voice 'Can you (please) [leave]'?