Question 1: the only record that you hace currently from the user is the sale receipt, you can create “customer” and assign them galleries but this is not an accurate record of purchase.

Question 2. this for site admins and if you want to secure multiple galleries for one customers and you don’t want to send multiple passwords

Question 3. are using paypal or google checkout? for paypal no, but there were a few updates on their system and I had not had time to test them. for google check out it requires SSL on your server

Question 4. the column are control via css or your thumbnail size, and of course your page width. The best option to change your css is in your theme styles.css just over write the plugin styles. this way you can upgrade the plugin without problems. Or you can disable the plugin css and completely write your own styles. settings > general > Use CSS