increasing IDL samples (d3d FFRender script) may reduce frequency of blotches.  no IDL precalc (IDL IC =100) may only reduce their size, but worth a try IMVHO.  drop shading rate to 0.2, incr. IDL bounces to 16, incr. pixel samples to 16, incr. refl. qual to 1.0, et al.

for those waiting to learn size of emitting surface in OP's render, it's bill's envsphere and lamp domes.

 

p.s. caisson, thx fr lnk to bill's BBeye.  I had crossposted in earlier thread.  my suggestion to bill is to incl. link in readme.