One way to check to see if its Inconel start at the valve head with magnet half way up the stem the magnet will stick.

I am definitely not an expert at failure analysis. But, looking at the pictures you posted, I noticed the area between the cylinders on the head. With the head bolted on, gasket in place, I cannot see how physically, the broken material from the valve, causing the area between the cylinder, to get damaged like it is. To me, it almost looks like that area got torched between the cylinders, probably led to the misfire you felt, introduced some water from the head gasket being melted, and thermal shocked the valve causing it to let go? Some of the fire ring/aluminum sent to the adjacent cylinder, causing the hammering marks to the quench pad?

Just throwing that out there. Its always easy to say a part failed, but I try to look deeper, so it doesnt happen again. Maybe some of the more experianced builders will weigh in on that.

That sounds very logical to me......

exactly..... I have gotten Inconel very hot and it didn't break, no idea how it didn't, Gellner told me he has yet to see one fail, not that it can't happen, just never seen it in all the years of building, mine had to be INSANE hot to do this.... but didn't break..

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I saw that material missing between the chambers which is confusing as ever.....

The heads that I've seen get torched normally have smoothed rounded edges (generic pic posted from google search), whereas this looks like it chunked out with the square/sharp corners.

Was the head stuck on the head studs and used a pry bar to remove? Pry bar marks? Looks like a lot of corrosion in the bolt holes.

Could very well be what happened

A couple of local engine builders thought it lifted the head. The gasket had the exact same impression but was not burnt out.
The head came off easily.

It looks to me in the pictures that there is carbon in the markings in the quench pad. I don't think that's from the valve in the other cylinder. Does anyone notice all the speckling to the quench pads on the other cylinders? Mayor do you have any pictures of the old piston tops or spark plugs? Did anyone happen to measure the ring lands on the old Pistons to see if the rings were shaking?

I have all of the old parts.

If you get a chance post a pic of the piston that was next to bad cylinder.