Yes you can install KODI and add NAS shares as a source, this would allow you to stream the content. You can also install Plex client on the SHield TV, however transcoding (if needed) still needs to be done on your NAS device.
I treated myself to the Nvidia Shield and watch with Wilmaa. It runs faster and more reliably than on some TVs with an integrated Android TV operating system. I think the same is true for Zattoo.
Actually only RAM (2 vs 3 GB) and internal memory (8 vs 16 GB).
However, USB might be the most limiting. You can't attach a webcam or USB gamepad to it.
The internal memory should be sufficient for the Plex cache (icons, fanart...), but could be swapped out to the SD card, which you should make sure is fast. For me, an XC Class 1 with about 10 MB/s write only worked to a limited extent, as it slowed everything down (not tested with Plex, but other apps).
Sure. The part is nothing more than an Android TV. I had a quick look in the Google store, BlueTV is there.
For motorhomes, I would personally prefer the Strong LEAP-S3, as it is powered by 12V.
The original FB is Bluetooth.
I use the TV's FB, which works perfectly with the Shield via HDMI-CEC.
All the functions of the Shield, calling up the App and operating it work perfectly.
I think the signals of the Harmony are also passed on to the Shield via the HDMI-CEC interface of the TV, except for the functions concerning the TV itself.
The remote control needs Bluetooth to communicate with the Nvidia Shield, i.e. Bluetooth cannot really be switched off. But the Nvidia Shield devices are very good, so I wouldn't worry too much about Bluetooth.
Yes, the Nvidia Shield TV can play everything. And it does so in very good quality. Be it what you get via Plex or via KODI, ISO files also work without any problems. Audio formats should also not be a problem, because everything that is possible today is supported by the Nvidia Shield TV 2019 box.
The only thing your TV needs is a digital input. E.g. HDMI or Displayport. A 2015 TV should have this, otherwise take a quick look behind the TV or google the connections of the model.