You're proceeding from a misunderstanding:

"This resulted in massive ranking-drops, etc. because (i) Google thought oldURL.html has exactly the same content as newURL, causing duplicate content issues, and (ii) Google did not transfer the juice from oldURL to newURL, because the 301-redirect was never noticed."

• If 301 redirects are in place, on a 1:1 page-by-page level, Google doesn't think there'a a duplicate content problem, because your server is telling Google that A has been superseded and replaced permanently by B.

• The 301 will be noticed, and "juice" will transfer. Just not overnight, and possibly not by a factor of 100%.

What you'd done sounded fine. Google will visit your site and index your new URLs, the redirects ensure that bots following external links to your site end up in the right place, etc. You could perhaps further expedite the process by submitting your old Sitemap alongside a new Sitemap, both referred from a Sitemap index, to be 100% sure Google hits all of your old URLs and follows their respective redirects.

The fact that you've rolled back won't have helped matters any I'm afraid, but shouldn't be an insurmountable problem. Just be thorough with your redirections, and be patient while things to settle down afterwards.

You're proceeding from a misunderstanding:

"This resulted in massive ranking-drops, etc. because (i) Google thought oldURL.html has exactly the same content as newURL, causing duplicate content issues, and (ii) Google did not transfer the juice from oldURL to newURL, because the 301-redirect was never noticed."

• If 301 redirects are in place, on a 1:1 page-by-page level, Google doesn't think there's a duplicate content problem, because your server is telling Google that A has been superseded and replaced permanently by B.

• The 301 will be noticed, and "juice" will transfer. Just not overnight, and possibly not by a factor of 100%.

What you'd done sounded fine. Google will visit your site and index your new URLs, the redirects ensure that bots following external links to your site end up in the right place, etc. You could perhaps further expedite the process by submitting your old Sitemap alongside a new Sitemap, both referred from a Sitemap index, to be 100% sure Google hits all of your old URLs and follows their respective redirects (but as discussed in comments below, this shouldn't be necessary - rule out other problems first).

The fact that you've rolled back won't have helped matters any I'm afraid, but shouldn't be an insurmountable problem. Just be thorough with your redirections, and be patient while things settle down afterwards.