Sadly I could not find your key for verification.
Yes someone else told me yesterday. This key server seems to have issues (even on the website, all search result in 503 errors). Maybe I should upload my key somewhere else.
Is it sufficient to mirror the gimp/pub/gimp directory (like we are doing at the moment) or should we mirror the whole gimp/pub area?
You may keep the same tree structure as you already had, no problem. gimp/pub/gimp is the most important part, and in particular the part needed for the download rotation (where tarballs and GIMP packages are).