Re: spike in heat- does it affect lettuce?
I started over on my earliest salad plantings for similar reasons. First they limped along in wet/cold, then they got a baking and then we went back to wet/cold. RE: the blazing sun stuff, erect a piece of shadecloth over the tender things to give them a break perhaps. You can use Reemay fabric and clothespins and old coat hangers or but “frames” from a garden supply place made for suspending shade and other protective fabrics.