Well.... that wasn't fun. Fixed the arcing issue by smoothing off all sharp edges and increasing the height of the inner PVC sleeve above the windings.
New $15 fuse in the multimeter, I then proceeded to measure current draw on fixed frequency (mid-frequency). Current draw figures at mid to full power matched those I had measured before with my 246/270 setup. Checked the scope trace at about 45% power and it looked okay. Comparable to what I had seen in other tests where I had been able to run full power.
I then went into midi mode and ran the first 20 seconds of "Beetlejuice" as I knew the current figures from those from my previous testing. First ran at half power to ensure all was good, checked the primary temperature for any excess heat, then ran again at full power. Everything appeared to be measuring the same. Visually, everything looked and sounded like previous runs with noticeable change to the spark intensity (not length as noted before) as I went above 50% power. So then I clicked stop, loaded "Paranoid", turned the power to full, then clicked play. All went good until 7 seconds in. All of a sudden... silence, except for the multimeter beeping like mad like last time the fuse in it blew. Figuring the damage has already been done, I remove the multimeter from the circuit and plug the mains back in. This time, all the onboard LED's fade off. Mains circuit breaker was tripped.
IGBT's and/or diodes are shot.
My very first oneTesla failure. Might celebrate with a beer Edit: So both IGBT's are dead and two of the four diodes. Inconveniently, I don't have any spare MUR460's so looks like I will have to wait until Tuesday to get them as they have to be shipped from interstate.
I'm tempted to reset everything again and continue with my known good "double toroid" configuration... provided I get no further catastrophes upon rebuild. I'm going to incorporate some TVS diodes across the IGBT's for added measure, but I think I will accept my oneTesla for how it was before with it's 6 turn primary and dual toroid. Yes the primary does get a little warm on full power runs of about 2-3 minutes, but everything felt "safer" despite that. I will proably just set up a cooling fan to keep the primary cooler.
Also, for curiosity, I cut apart the ceramic fuse I used on my main board. The insides showed signs that there was a good arc inside there which is what led to the circuit breaker tripping. I reckon if it weren't for KevinNewZealand's post about his fuse exploding which then led me to change out my fuse, I probably could have had the same explosive catastrophe on my main board.