Drew,
To answer your first question and especially for travel in May, I would highly recommend pre-booking lodgings in the Cinque Terre. That's the spring "shoulder season" and it can be busy there. While you might be able to find something when you arrive there, you may also have to spend some time and possibly money in trying to find a place to stay.
Where in the Cinque Terre - technically Corniglia is the middle village, but Manarola would be a good choice. Most of us here have a favourite of the five towns and my preference is Monterosso. It's the largest of the five and therefore has the greatest number of lodgings, restaurants and other tourist amenities. It also has the best beaches of the five towns. Most of the lodgings in the other four towns are small B&B's or private rooms, while Monterosso has more "conventional" hotels as well as B&B's (many of which serve a nice breakfast).
It's very easy to travel between any of the five towns using the frequent local trains, so no one village is "the most convenient spot" for getting around. One point to keep in mind that not all trains that pass through the area stop in all of the villages. As Monterosso is the largest, my "hunch" is that a greater number of trains stop there. The boats will probably also be running in May, so that's another possibility. Taking the boat from Monterosso to Porto Venere (south of La Spezia) is a nice day trip, and it provides a nice view of the five towns from the water. The boat stops in all the towns except for Corniglia.
One of the rituals for many visitors to the Cinque Terre is to hike the Sentiero Azzurro trails between the towns. At the moment, only the segment from Monterosso to Vernazza is open, but that may change in the near future. Check with your hotel when you arrive to find out which of trails is open.
