Hi
1) Is it a prerequisite to use maps, or can they be omitted from listings and the site as a whole?
You can hide/disable the maps.
2) In the search, is it possible to remove the keyword search altogether and use categories instead?
You will need to do this manually, the theme does not have a function to remove the search bar or modify it.
3) Images - can a restriction be placed on image uploads by an advertiser to - lets say - a maximum of five uploads per post?
By default all listings can have up to 5 images.If you need to change that you will need to modify the theme.
4) Advertising packages - can these be removed completely, allowing all ads to be free, but with optional paid-for enhancements?
You can turn off charging for posting a listing completely or create plans that are free and add 'featured addons' which make the listing featured on the home page or category.Here is a relevant link
http://docs.appthemes.com/tutorials/...pricing-plans/
5) Is it true that there are AppTheme advertisements installed by default, and if so, can these be removed?
You can place ads in various places in the theme, when you install it the first time you can remove all the default AppThemes ads and place your own.
6) Image galleries within a user posted listing - can these be edited to remove the lightbox, and/or customise the style of the gallery?
All code is open for modification, you can modify the theme and remove the lightbox if you prefer, but that requires programming experience.
Let me know if you have any more questions.