It would be possible to show in the edit gallery page the Custom fields and all the other option for a post?
yes, you can edit the custom post type, or use other plugins to modify the post type

your menu problem is a style issues also. give me a few days to take a look at the styles.

if you want to donate you to the plugin there are a few places where you can find the link. The plugin settings page, in the footer of this website or the right-hand side of the plugin in wordpress http://wordpress.org/extend/plugins/image-store/