You're making a good case, but this isn't part of it:

"It was 190 degrees upon contact. Her grandson pulled over to the curb after they bought the coffee. "

You don't know this. Heat doesn't stay at a constant. Exponential Decay and all that. Now it may have been 185 degrees or even 189. But it could have been 180 degrees.

Neither you, nor I, know that it was 190 so to keep saying it was is defeating your argument.

Neither you, nor I, know how long it took before he drove off and then pulled to the curb. We don't know if they had a conversation first, or what.

We also don't know how much the temperature dropped once it spilled OUT of the cup and until it HIT her skin. Depending on the outside air temperature this could also be affected.

My point is there are a lot of variables. We also don't know how long it stayed in contact with her skin. The possibility that it DID take 20 seconds is there, right?

All we have is her word that events happened the way she said they happened.

And still - she balanced it between her knees. That's just stupid.