I just tried with the three settings increased as shown attached. Not sure if all of them are needed, or to be increased that much...
Didn't take noticably longer to pre-compute the cache, but the probelm's gone and the rendering also clears with MUCH fewer samples.
Shadow does OK. It's caustic in reflection that's spilling over, so Caustic Light Cache / Lookup Radius of 1cm is your solution
Also, use thickness/solidify your geo in future to prevent other issues - while modeling have physical correctness in mind