Hi Martin,
As far as I know, we can not apply usage rights only by using LiveCycle designer or AEM Forms Designer. Reader Extensions is a server module and you need licenses to use that. I am not sure what kind of support you have from Adobe. You can reach out to them and figure out the options of using the Reader Extensions in AEM Forms designer.
Earlier you might be using Acrobat Professional along with LiveCycle Designer and do the Reader Extensions. You can do partial reader extensions with Acrobat Professional.
If you want to use LiveCycle Designer itself in Windows 10 as a work around you can still do that.
To install LiveCycle Designer in Windows 10, you need to run the LiveCycle Designer installer file in the Compatibility Mode of Windows 7. This will install the LiveCycle designer in Windows 10 and keep using it without any issues. I have done this on my Windows 10 machine at my workplace and been using it for almost 2 years. I did not see any problems with the forms using the LiveCycle Designer in Windows 10.
Thanks,
Manoj