Hi jessbky,
I am a graduate of the post bac program so I hope that it's not too late for me to answer your questions!
1. For the letter location, I would just give a general address to maybe the department that the person works in. Say it was a biology teacher, just give the address of the biology department.
2. You really don't have to provide too much information for the Experiences section. They just want to see that you have been involved, what you have been involved in and to what degree, and for how long. You will have an opportunity to elaborate more during the interview!
3. This questions is more asking for an explanation of any time that you might have taken off. So say that you took a year off to help out a family member or you just decided you needed a break from school. They just want to fill in any gaps in your education to see what you were doing during that time. In addition, I would also include an explanation of what you have been doing if you have already graduated from your undergraduate program.
4. Submitting your application by the deadline will put you in the same pile as all the other applicants! It is not like medical school admissions where applications are reviewed on a rolling basis. They collect all the applications, then review them and offer interviews. I was in the same boat you are...I am from Michigan and didn't really consider California post bac programs until January. As soon as I found the program, I had something like 15 days to get it all put together, and I got in! I don't think you should worry about submitting it by the deadline.
Let me know if you have any other questions! I would be happy to answer them all!