Thanks for your interesting post. Glad to hear that I'm not the only one... Got the same Windows 10 build (1809).
I didn't test your solution yet. First of all, I've got some questions about it:
(1) Did I get you right that the Opus 12.10 installer contains Opus 12.11 in fact? Sounds really odd... together with the fact that both files have (exact?) equal size.
(2) In 12.10 file properties, the compatibilty checkbox was already activated by GP Software?!?
(3) Is it possible to install 12.10 version without marking that checkbox? (I don't want to run Opus in compatibility mode and the support team told me not to do).
(4) Did you have the same problem that Opus 12.11 proposes wrong installation folder (despite 64-bit OS)? The same with 12.10 installer?
Might be a hidden bug in 12.11 installer, as supposed in my last post. Or one of these mystic faults caused by Windows 10 October Release (according trade press, there are a plenty of them).