Hitachi TV (LE55V707) Roku3 > HDMI > TV ( no AVR)
.. … All your directions are for the reverse of the issue I am describing…...
Actually, from your description, the issue seems in reveres.
As you seem to be under the impression that the TV accepts/decodes/process Netflix 5.1Dolby Digital Plus (5.1DD+) and outputs the sound thru its two speakers (w/ great audio) but when you switch netflix to stereo audio began in half speed.
Q- How you determine if the TV actually decodes 5.1DD+? or does it try to transcode it to 2-ch PCM? or …..?
Keep in mind, Netflix [5.1] titles are encoded in 5.1 Dolby Digital Plus (5.1DD+) and not in 5.1 Dolby Digital (5.1DD legacy).
The idea here is to completely avoid DD+ signal with your TV and reset TV settings (you experimented too much already)
So, try to perform step 1~5
(above), stick with stereo only and avoid feeding DD+(plus) audio signal to your TV (that maybe causing the issue, since your TV doesn’t decode DD+).
& report back to help find out cause/fix.