Hi and welcome,
First of all, with Cubase 10.5 license you could start and run Cubase 9.5 too. So in the works case, you can install and use Cubase 9.5 too.
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Yes, there is backwards and forwards compatibility between versions 9.5 and 10.5. In both versions, you would use common CPR file.
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If you would buy a retail version (the “full” version), the USB-eLicenser comes with. If you buy any update, then you have to buy the USB-eLicenser extra.
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Yes, Cubase Artist is also compatible. Be aware, the Pro functions wouldn’t be available in the Artist version. So you could have missing plug-ins issues, for example.