Please understand that when a device does not support CEC, only the control bus (explained below) will not work. It will not affect any other aspect of the HDMI communication. CEC will not stop or disturb audio and video communication. In your case, the STB is a source. What we are trying to resolve here is the ARC between TV and AVR, and this has nothing to do with CEC. Once the audio and video data reaches the TV, what the STB does is meaningless.
For ARC to work properly, the CEC/ARC has to be implemented in Toto by the device. This will mean additional costs in design, testing and manufacture. For example, for the TV to use ARC properly, it should implement a switch for all HDMI connections. As far as I can tell, this has not been done by TCL. As long as you do not disturb the connection between the TV and AVR from the ARC HDMI channels, things work properly. The minute you choose a different HDMI on the TV, the ARC communication is lost. The video is routed to the TV screen, but the audio is not routed to the ARC channel for onward routing to the AVR. As I said before, this needs a hardware and software switch inside the TV.
For Your Information:
CEC is a single wire bus that is built into the HDMI communication. The bus is the basis of automatic control in HDMI interfaced system. Any device that uses HDMI can have CEC built in. When a device does not support CEC, only the control bus will not work. It will not affect any other aspect of the HDMI communication.
The basic technology of CEC is based on the SCART interface that was used originally across many consumer electronic devices. HDMI borrows and improves on this to enable AV products to identify one another and communicate with each other. Upto 10 devices can be controlled through CEC enabled HDMI. For example, if you switch your STB on and play something, it should switch your TV and AVR on, route the video to the TV and the audio to the AVR.
"The CEC protocol includes automatic mechanisms for physical address (topology) discovery, (product type based) logical addressing, arbitration, retransmission, broadcasting, and routing control. Message opcodes support both device specific (e.g. set-top-box, DTV, and player) and general features (e.g. for power, signal routing, remote control pass-through, and on-screen display)."
It is upto the device manufacturer to decide how much of CEC he will implement.
