Using HDMI-CEC but don't turn on/off TV

Theoretical it's possible, but seems depends on your existing equipment and cabeling. Within the CEC adapter settings you can configure what should happen on power on/off/standby events of KODI. The issue could be if you need to signal KODI as "active source at startup" to get the TV remote control working for CEC or to switch to the right input, this triggers some TVs to power on.

Not possible. The signal chain is RC -> TV -> RPi. Obviously that doesn't work without TV. Try an IR receiver on your RPi.

You can configure the RPi to not-wake the TV over HDMI-CEC when the board is turned on, but if the TV is off, the IR/BT receiver on the TV is also off, so the TV cannot send remote commands over HDMI-CEC to control the RPi board.