Please include a url to your site install of the classifieds and test user login
1. Your error suggests you have not placed the correct path to Imagemagick in the config settings. Make sure if your using GD2 as you state in Admin => Global Options you select GD2.
2. Is this the whole error? I would suggest you set your uploads and data directories at every level beneath them to 777 permissions.
3. I would have to see what you mean because writing to database tables and some extra slash are totally different things so I am lost in what your trying to report there.
4. If your server does not support spider friendly urls in classifieds you can turn them off in global options in admin. That is why most features have an on off setting not all server setups allow that setting.