Correct.
A graphics card will be of zero help for your installation.
Transcoding video using a GPU, whether embedded in CPU or discrete, requires a Plex Pass. As you do not have a Plex Pass, all transcoding will be performed by the CPU.
Note also that transcoding 4K HDR in Plex is a horribly bad idea. When transcoding, Plex converts all video to H.264. Furthermore, Plex does not currently support HDR to SDR tonemapping. As a result, transcoded HDR material generally looks very poor, with washed out colors.
Yes, with the caveat that you cannot use subtitles when this happens.
On Android TV devices, including FireTV devices, when audio is transcoding, AND subtitles are enabled, the video will transcode. Plex does this to keep the video, audio, and subtitles in sync with each other.
Blu-ray discs with TrueHD audio also include an AC3 version of the audio track. You are much better off using the AC3 audio.
Yes. It will just sound like audio from TV speakers.
Good thread with additional information
