Solved - USER ERROR
I realised last night that this was down to me not clicking Apply after making my changes and pressing "ESC". In my defence ESC does exit other screens in the game without abandoning changes so it might be useful to include a warning if there are unsaved changes on the keybinding change but I will suggest this on the dev forum.
Smuggling takes a bit of effort as well as some luck.
First you need to understand how detection works, your ship's heat signature is above your fuel gauge. The bigger it is, the farther you will be detected from. Turning on Silent Running will cap this detection range at 500M, but it will heat up your ship very quickly, so use it sparingly or pack heatsinks
You always need docking permission and landing is as you would normally. You should make sure that the system you are heading to has a black market, though. (the commodity market won't buy illegal goods)
There are no modules/internals that will hide your cargo, but you can go for ships that have a low default heat signature like the Cobra, and turn off unnecessary modules to lower your heat output.
Try going for outposts instead of starports, as they generally have much less enforcement hovering around. If you do find yourself at a starport, lining yourself up with the mailslot while out of range or flying very close to the starport will hide you much better than just moseying on in.
If you are getting scanned, don't panic. It takes a short while for the scan to complete and once you enter the mailslot, the scan is broken. Just pick up the pace a bit and get in there.
Best Answer
You can have two keys bound to the same effect.
In your example, both [Down Arrow] and [Mouse 1] will activate Thrust Down.