CEC is one of those technologies that should work, but doesn't. My old TV did not pass through channel up/down and instead used them to switch back to aerial requiring six (yes, six) keypresses to go back to HDMI. I miss proper old remotes that had dedicated buttons to go to an input.
CEC mapping varies with pretty much every tv (even between models) and every device connected to the tv. There is a keyboard mapper in Kodi's addons that allow you to map keys to functionality. In general, I think you're better with an "air mouse remote" off ebay, which has a remote on the front and a keyboard on the back.
The reason I suggested a laptop is that a lot of issues might just disappear. Connecting both tuners too might reduce some problems. The laptop doesn't need to be on all the time since there are packages (such as turnoff) to suspend when not recording.
As they said in Jurassic Park, the problem with running media on the Pi is that people "were so preoccupied with whether or not they could, that they didn't stop to think if they should."