Hi Thomas,
1) is it possible to let users upload images to a post's gallery (not just the featured image)?
Yes, it is possible within CRED, I suggest you try create a custom image field using Types, CRED will aoto generat the image field for you.
2) is it possible to populate a dropdown select menu with the children of a parent taxonomy selected in another dropdown select menu?
No, it is not possible within CRED.
3) – is it possible to use the WP post status as conditional (e.g. if "Published" -> display "edit" link / if "Pending" -> Display "Waiting for Approval" info text)?
you can try CRED action hook "cred_save_data", Hook to do custom action when post data are saved to database.
http://wp-types.com/documentation/user-guides/cred-api/
create a dropdown menu manually in front-end, when user submit the form, use this action hook "cred_save_data" to modify post status.
4) – is it possible to assign an auto incremental number (eg. Listing001, Listing002 etc. or Premium001, Premium002 etc., used as unique post ID) to a post created via a form?
Every post created by CRED will be auto assigned a post ID, which is unque, if you need modify it, you can try use the action hooks "cred_save_data" to modify to what you want.
5)it is possible by these:
a) hiding the toolbar via CSS
b) create a textarea for post content manually in front-end, replace the default rich text editor for the post content
Regards
Luo