All attachments uploads are handled by the WordPress Media Library, and plugin uses native WordPress upload functions. When file is uploaded it will be available through Media Library. Consult WordPress documentation about Media Library requirements.
Yes, since GD bbPress Toolbox 1.4. But, when updating, you must recheck all plugin settings depending on roles, and select roles you want and save. When dynamic roles are used, normal WP roles are suspended.
Yes. If any of these plugins (GD bbPress Attachments, GD bbPress Tools and GD bbPress Widgets) is active, GD bbPress Toolbox will disable them on activation.
Yes. Each site in the network can activate and use the plugin.