There are most definitely elliptical orbits. Check the eccentricity field in the system map.
Canned elliptical obits, obviously - you can't enter into
any proper orbit in ED, let alone an elliptical (ie. natural) one, in which your speed would necessarily rise and fall outside the narrow range ED is capable of handling.
I believe flight assist has something to do with this. Scott Manley has an old video showing him orbiting a planet with FA-off after reaching escape velocity.
Plus these are airless planets we can land on, so there's one not-unsubstantial force being ignored in the OP that should be taken into consideration.
What that video shows is clearly an artificial effect - achieving a perfectly circular orbit (ie. in which the speed is truly constant, as in that video) is practically impossible, especially on the initial attempt - without calculating everything to five decimal places, you'd inevitably need to be making lots of corrective burns throughout the lap.
Even a geostationary orbit (so not moving relative to the ground) would see you moving at far higher speed relative to space itself, than ED is able to cope with. And if you're thinking that regarding motion in relation to space itself is just idiotic gibberish, unfortunately you'd be right, but regardless, that's how it's implemented in ED - your ship's speed is measured relative to the coordinate space of the instance around you (part of the reason why we're stuck with space speed limits in the first place).
So while in principle it seems it should be possible to orbit a sufficiently low-density object within the confines of ED's space speed limits, we're talking a
very low density body, with extremely weak gravity, and thus it would be so slow
and low as to be all but impossible. I wouldn't hold my breath for a convincing demonstration from either ground-launch or entry from space (or warp, in ED's case), from any player.. It's a more a matter of fundamental constraints on the game engine and piloting freedom, than anything to do with skill...
ED just isn't physically capable of seamless space travel. It
needs to generate instances, which
have to be linked via
another "supercruise" instance.
Of course in reality, any gravity well at all is going to trap layers of gas, UV radiation (even from distant stars) will cause mineral vapours to slowly accumulate even in the complete absence of any geothermal activity, and ED's ships are so hopelessly crippled they'd be trying to orbit well within that gas envelope..