...fwiw...I work in projects all the time with multiple instances of Melodyne ARA 5/studio active without any trouble, I'm not sure what the difference/s might be...but I take your post as a cautionary marker...hth.../s~
I "think" you may have missed the actual issue. It's not working with multiple instances of ARA as long as it's the same ARA plugin. It's working with mixed instances of ARA (different ARA plugins for different vendors in the same session) that is the issue. Multiple instance of the same ARA plugin shouldn't be a problem. You're in the clear.