HAHAHA QUARTER ONIONS, WHO IS THE BOSS NOW?!
So, I finally managed to wrangle rotation into submission through the judicious intake of coffee and a liberal application of rolling my face on the keyboard out of sheer frustration.
Now, clicking on a location in space will make the ship turn to face it, and turn towards it. I just need to change the rotation from direction rotating via transform.rotation (naughty!) to using RigidBody's AddTorque. (The movement is already using AddForce).
I also added a spiffy header bar on top that does absolutely nothing (for now), as well as quick and nasty Text Meshes to the planets to display their names. The scaling on their orbit circles is just as suspect, it lies close to but not exactly on their centers which is really annoying me.
I also managed to find whih prefab/script was giving me that damn "the referenced script on this behaviour is misssing unity" error (I forgot to Apply changes to the prefab on my player controlled ship).
I need to make a spiffier course plotter. Maybe something that shows the ship's current vector and the destination one (which would have made implementing rotation easier in the first place but I digress). As well as a nicer off screen bracket pointer. And make the engine trails be dependent on the force (so thrusting gives longer trails than turning). And and and...