Well...I did a bit more playing around after the post.
But in response to a few of your points.
I'd want a redirect, users should be sent to the new address, and the address bar should reflect this. I realise there is no R=301 at present, but there should be one on the finished article.
A bit of reading up on Regex's in mod_rewrite, and I've learnt what the QSA is, and have got rid, likewise with the NC.
And I like bonus points ;) Even though I'm not sure why there's no trace of .* and why I haven't included it.
Back to some testing I did.
I've narrowed it to this bit of code, which thankfully (in case you haven't noticed) hasn't caused the destruction of the universe:
RewriteRule ^galleries.example.com/([a-zA-Z0-9-]+)/([0-9]+)/?$ http://www.example.com/gallery/%1/$1 [L]
The original subdomain is currently pointed away from the normal server (another DNS), but will be changed back. I've done some testing on another subdomain (same DNS) to see if it works or not...and it doesn't.
According to online htaccess testers, there doesn't seem to be anything wrong with the code, but yet a redirect isn't happening...so I'm a bit stuck as to what to do...