1. What DW extension should I use to create the upload form? Data Assist or Form Builder? 
since you are inserting data to the database, use data assist to create the insert form.
2. What is actually stored in the database, photos & documents or just links to photos and documents? If only links are stored in the database, where are the photos and documents stored? Will I need to write scripts to make the links work?
the name of the uploaded file is stored in the database
3. How many Browse & Submit buttons can I put on the same form? Can you use the file upload behavior with a data assist insert page? Will the browse, submit & insert buttons be compatible?
you should only have one submit button on the page to submit the form, but you can have multiple file fields.
4. Have I overlooked the instruction manual? I cannot find any answers to my questions.
On the Data Assist support page, there is a tutorial for "How to apply Digital File Pro to DataAssist Insert and Update pages" (Digital File Pro is the same as the File Manipulations behaviors in CSS Form builder):
dataassist/
