The sprite is not mirrored if you use a negative rotation speed. It just uses the same angle as if it had a positive rotation speed.
I believe this is by design as it matches other features in Construct, like the Platform behavior. If you intend to flip the direction of the sprite, you can use the 'Set mirrored' action. For example a typical platform game will use the 'Set mirrored' action as the player presses the left and right movement keys - the behavior does not control that for you.
I now got the code to work using set mirrored action -- however, quite like my earlier work around -- where I set a flipped frame, to rectify the flipped image -- the set mirrored is visually noticeable by making the sprite blink.
I still believe that there is a case for having a dedicated property for orbit action to make it circle counter clock wise without the need for negative speed (which is semantically unclear) and related work-around.
If this clashes with platform behavior then perhaps the design there is not optimal either.
And, if two designs need to co-exist, the perhaps two orbit plugin variants are needed -- or a third property that establishing default behavior such as now, and counter clockwise behavior such as I now need for here.
The way it is now, its unfortunately, not usable since it significantly reduced the visual quality of the end results.
It would be a pity if due to that i need to try to work things out with vector math, which has its own problems (e.g. how to set and adjust speed consistently across computers, if speed is angular displacement).
And, overall -- what I really like about C3 is that it makes such things really simple to do -- so, just to get counter clockwise orbit -- shouldn't require one to drop down to complex circular vector math.