Unfortunately, anyone who actually knows the answers to your question is under an NDA and cannot discuss any release schedules.
Raw support for the D7500 should be coming soon. Whether it is a 6.x release or 7.0 no one can say.
Or can I always update Camera Raw on an older LR version?
You can't update Camera Raw in older LR versions. But you can use Adobe's free DNG converter to convert raws from newer cameras to DNG format and import the DNGs into the older LR.
My suggestion would be for you to wait and see if there is another update for Lightroom 6 or if there is a release of Lightroom 7. In either event, you won't be able to update an older version of Lightroom with the Camera Raw plug-in because you cannot update Lightroom that way. The Camera Raw plug-in is for Photoshop and Photoshop Elements only. All of the Camera Raw technology is built into the Lightroom program. If you choose not to update your version of Lightroom there will always be the DNG converter provided by Adobe. That is the free alternative for those who want to stay with older versions of Photoshop and Lightroom.