San Diego is not known for its public transportation unfortunately. Once you're on Coronado island you can walk or bike around to local restaurants and activities. But you will need transportation to get to the wedding venue. If you rent a car I suggest getting a mini van or SUV and sharing a vehicle with a friend or family member. As you'll need it for some things and won't for others. Uber/Lift etc are VERY accessible in San Diego if you prefer that method.
Lucky us... it's generally absolutely beautiful. A high of around 75-80 and low maybe in the 50's. Always bring an extra layer or two but usually it's pretty perfect!
There are hotels everywhere! Don't feel like you have to stay on Coronado Island! It is the most "walkable" for things to do but there are a ton of options. I would use the old "google" to find what you're looking for!
There's a shuttle going to and from the Lakehouse resort in San Marcos to the Wedding venue, Twin oaks gardens. The shuttle will run from 11 am - 11 pm and holds about 55 people.