I experience the similar issues. Not with rasplex but with PLEX Home theater. To me it works if I connect the Mac directly to HDMI1 at the Samsung-TV. (using pulse eight HDMI-CEC adapter since the mac does not support CEC out of the box)
However I want to 1. connect the mac to a AVR first and then to the SAMSUNG at HDMI2 connector. The reson for this is obvious I want to use my receiver and also want to use the ARC (Audio Return Channel) on my Samsung which is on HDMI2.
If I read XMBC posts it looks like there are settings in XBMC were you can set the HDMI input on the TV and also the virtual input on the AVR. Is it possible to find them in plex and modify manually perhaps?
I have a feeling that I would get it to work if I just could find the settings file.
The same would probably go for rasplex as well.
Or I am completely off here...?
Any ideas from all you brilliant people out there?
I also had some Problems with this TV-Remote over HDMI-CEC. In my case i tracked it down to the HDMI-Cable! The one was a noname and with that it was not working. Then i used another which was a Amazon Basics HDMI-Cable and voila - it was a charm!