I think I have posted about IR duplication among brands- I had a problem when I was trying to use IR to control a Digital Watchdog DVR using the IR receiver that was available, rather than the Harmony hub. When I used one command for the DVR, it changed the Panasonic commercial Plasma monitor to 'Setup lock'. I had no idea why it would do that, so I called Harmony tech supoprt and was asked to learn a few commands for the DVR in real time, so they could see the code.I was told that it affected both because they used a Toshiba IR control chip- the number of manufacturers of those was small and manufacturers of finished products would use the chip of their choice. The codes were unnamed, so they could be used for any button on the remote, even if it corresponded to a different command on another product's remote.

A lot of cheap products use generic NEC codes with very poor code rejection - I've had all sorts of weird interactions over the years. However, Marantz uses RC6 codes which are highly unlikely to be utilized by anything without a Philips-based background, and aren't likely to interact with any other code format. TCL apparently does use NEC format, but unless their code rejection is absolutely atrocious, I would learn more towards an ARC/CEC issue.

First, disable all CEC on both the Marantz and TCL. If the problem goes away, that's your issue. I sometimes get weird interactions even between Marantz and Sony, and rogue devices in the HDMI chain can also mess things up (for example I've had a media player that insisted on always changing the TV to the wrong input).

yes. You may need to use multiple code sets to get the desired inputs but yes the codes are all the same as long as your amp supports the correct inputs. You just have to play around with it a bit.