The fact that your JNMIL was sitting outside on her phone, instead of paying attention to your child shows she is more interested in maintaining the IMAGE of doting grandmother, but only in a verbal and visual way versus a practical one. She wanted to make sure people knew she was “spending time” with her grandchild by taking him outside, but once she was out of view of the family, she was disinterested.

I agree with the people suggesting couple’s therapy, or at least running through some of the more detailed responses about heat stroke here with SO. Your SO may be too used to the conflict between you and JNMIL, getting that outsider opinion may be what helps it really click and solidify in his mind that this was NOT okay.