Can i boil the full 5 gallons with a partial mash recipe?
Absolutely, this is recommended, just not always possible. Although you'll want to include loss from boil and hops (which I'm sure you considered). And Downinit makes a good point. Could take awhile without a hefty immersion chiller or something of the like.
And if I can, should I sparge with more than the 1 1/2 gallons the recipe calls for?
You certainly can, but it probably won't raise your efficiency any, so I don't recommend it. There is also the worry of oversparging, but it is very unlikely with your method.