Hey @Loeter , welcome to backstage! 
I’ll try to answer as many of your questions as possible:
Polyend has stated before that the Play+ does not mean the end of Play, so you can expect further Firmware Updates for it. As you said, obviously not all the new features will make it to the Play due to hardware limitations and at this point it is unknown what that will entail.
But what i am certain of… is if the hardware can handle it, Polyend will not gatekeep features from the old Play. For an overview what people are wishing for, check out the Play Wishlist.
Polyend hasn’t stated anything specific as of yet, so we don’t know.
Another thing to keep in mind for you. Getting a Play essentially gives you the ability to upgrade to the Play+ at a later time, if you feel like it. Polyend has stated that the Upgrade Path is not a timelimited offer.
It does not.
Currently it is not possible no. There is however a wish awaiting votes, that Polyend has approved, specifically for this.
Variations are not chainable no.
If you got anymore questions let us know. Maybe if you add or describe the context on how you are planning of using it, we can give you some more details. 