Hi Ben:
That's a nice, neat coil layout your have - I tip my top-load to you ;-)
My comments and suggestions are:
1) On the NST protection circuit, replace the RF chokes with large,
ceramic-cased wire-wound resistors, 500-2000 Ohms, 50+ Watts. It has
been shown that L-C filters as you have actually make matters worse,
creating their own resonances. I suspect that's why you saw the
flashovers on the caps. I know - they won't fit into the footprint of
your chokes :-(
2) Your doorknob tank capacitor uF value sounds good, but as you said,
their 20kV rating sounds too low to be safely used with a 15kV NST. I
see that you found them to get warm after use. Do you have a
capacitance meter to see if their value drifts when warm?
3) If you're using a 15/60 or 15/90 NST, you should be able to use a
much larger top load than a mixing bowl. Could it be that you just
didn't have enough primary turns to tune a larger top load?
Regards, Gary Lau
MA, USA