How do I restore this deck?
Before you start with a sander, I would try a power washer.  These can be rented from a big box store for a day or two. If you have several projects, consider purchase. You do need to be careful since the force of the water is strong enough to cause injury if mishandled. Also, do not try to powerwash very soft woods such as redwood or cedar.

It is amazing how much embedded grime and algae the washer can remove. If you still need sanding before refinishing, the amount of sanding will be greatly reduced. You also may find you can use a penetrating opaque or semi-opaque stain rather than the heavy coating type finish. Small chips or divots can be filled with exterior wood filler.

As to the swelled boards that eliminated drip joints, if this is only in a few places and there is no pooling water, this should not be a problem.  You do need to sand down any spots that have raised enough to be a trip hazard.