1) Yes you would need to customize the default theme in order to change the way in which the ad posting currently works.
2) By default, all users are required to register in order to post an ad, so changing this to allow non-registered posting would also require customization.
AppThemes does not officially support customizations, so there are no existing tutorials for making these modifications, so working out "how to" would be up to you, although as a customer you can seek community based support for modifications in our forums. Alternatively, we do have a
certified partners page where you can find developers for hire. If you make contact directly with an outline of your requirements, they can provide a quote for the work. Thanks.