Responses to your questions:
1. Yes The NEC actually does not limit how many lights can be run off of a individual junction box, it sets requirements for j boxes. Look at NEC 680.24 it sets forth the jbox requirements. Additionally, the Jbox like all equipment involved in a pool must be listed. So the J box must be listed for three lights. The Box Kadavis shows above appears to be listed for 3 lights.
2. Yes you can run plumbing and electrical in the same trench. You must be at least 18" below finish grade.
A couple of comments. First pools are not normal when it comes to electrical, equipotential bonding is strange, and other concepts are different. So its important that even if you are an experienced electrician and have yeas of experience that you read NEC 680 and understand what they are talking about.
You should also read and understand this:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwis1LqJxsvPAhVGVWMKHbI3DOkQFggnMAE&url=http%3A%2F%2Fwindsorlocksct.org%2Fsite%2FSwimming_Pools_and_Spas_2014NEC.pdf&usg=AFQjCNGR0vx2GBUcgEQRp1iMyMGO7Nw3dQ&sig2=X44pNHUeSPX0Hn0quOZ7wg
That is designed to train electricians but it sets forth many of the concepts you need to understand.
Additionally you should look at some local pools and see what is expected in your location. Talk to inspectors. Many counties in California will not accept a single #8 wire around the pool, they require some sort of metal grid under deck surfaces. There may be other local practices and customs, like using a sub panel as a disconnect. California inspectors are trained and have certain expectations at each stage of the inspection process so you need to be prepared for what they expect. So its always a good idea to go down and have an over the counter talk with an inspector when your plans are approved.
Finally, California is currently using the 2011 version of the NEC (Google Calif electrical code for an online version). California will switch to the 2014 version in January. There are some minor changes.
Good luck