Thank you for the reply, Ray!
I got the parts replaced last night (my family is lucky enough to have a nice iron which makes it easier).
The triac on the expansion board has the 3 pins as well as a larger pin on top…perhaps a mechanism for sinking heat?
It looks like the larger pin and the center pin are connected in the part. (page3 of the datasheet)
When we removed the burnt triac, it appeared that the pad for the center of the 3 pins was gone/vaporized. However, the pad for the larger mount/heatsink pin appeared to be fine. The connection/trace to the output pin also looks to be intact. So I think even with the smaller/secondary pad gone, it should still be fine?
In my case, I really only need ~12 outputs, so I could just avoid output1 of the expansion board if it doesn’t work.
I hope to get it hooked up sometime this weekend to try things out.