Re: They do self-heal...

>capacitors tested were all WIMA MKS 4 (K4), rated for 1uF at 400V_.

These would be a bad choice for an MMC cap.
They will as you have discovered take the voltage but
they won't be able to handle the high frequency current.
Using a lot of little caps in series and // means that the current is
amongst them. If I have 10 caps in // with a dv/dt rating of 2000V/uS then
the group can handle 20000 V/uS, a long single string of 1uF caps would
have a dV/dt rating of say 1000V/uS.

Alan Sharp