QVP:
My QVP was also v11 and you're right, it refuses to install on Windows 10.
Looking at Avantstar's website, they still advertise integration with Outlook, IE and File Explorer, so I am not sure people are correct that they have removed the preview handler aspect of QVP from newer versions, unless the webpage is wrong or they are explicitly blocking certain programs from using it.
On the other hand, I see it is now sold as a $99 annual subscription (with no upgrade pricing from previous versions, to make matters worse), so I don't blame you for not wanting to upgrade it. They must be focusing entirely on the business market and shutting out typical home users. A shame.
FoxIt:
I just installed FoxIt Reader from their website and it still includes a preview handler which works in Opus.
When you install FoxIt, make sure not to turn off the Shell Extensions option in its installer. (According to Why the Windows Explorer PDF preview stops working? on their website.)
You can check if the FoxIt preview handler is installed, and if it has the .pdf extension assigned to it, from Opus by going to Preferences / Viewer / Plugins, selecting the ActiveX + Preview + Office + Web plugin and clicking Configure.
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If it is listed there but doesn't have .pdf assigned to it, you can assign it there.
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If it isn't listed there, try reinstalling FoxIt Reader and check the option mentioned above. Make sure it is FoxIt Reader and not one of their other tools as I don't know if any of the others include the preview handler.
Failing that, there are several other PDF readers out there with preview handlers as well, but the FoxIt one still seems to work fine to me.
WordPerfect
Writing our own WordPerfect viewer would be a huge undertaking, and would probably take us years to do. It's not a trivial format to decode or render. It's really up to Corel, who make WordPerfect, to provide a preview handler for their software's format, the same as Microsoft provide them for the main MS Office apps, or to provide a component which people can use to create a viewers themselves.
Edit: You mentioned here that WordPerfect was including preview handlers in newer versions? Did that turn out to be incorrect?
Then I learned that WP-X8 suite which I did purchase has preview handlers built in for all their file types.