Thank you both for the info.
I firstly tried starting clean, and gave the MECOOL device a go. Firstly I had to program the power off/on toggle with the original remote. But once in Kodi, no buttons worked at all.
So onto the next method, adding the Minix NEO U9-H as a device into the Harmony remote. I can't believe I couldn't find the damn thing before. I was using all sorts of search strings, but in hindsight, not the most obvious one!
Anyways- once configured with this device in the Harmony activity, I could power on and off OK. Various keys didn't work though, like play/pause, page up/down. Most of them I programmed with the Keymapper add-on in Kodi.
What was throwing me off though was 2 buttons in particular that were very stubborn. Info and Guide.Leaving them with their default config from the Harmony setup just wasn't working, Kodi and even the Keymapper addon itself just wouldn't recognise them.
I ended up running Kodi in debug mode and seeing in the error logs, that long story short, the particular codes that they were pushing to Kodi wasn't recognised by the kernel. So for "Guide" and "Info", in the Harmony software, I re-mapped them to a couple of random number keys, "6" and "4". Then via the Keymapper addon, remapped them to the relevant "Info" and "Guide (for my case, codec info) functions.