Your hen's put herself into "lockdown" the last 3 days. That's why she won't get off. First, if it's the beginning of lockdown, Day 19, you could take her off for 5 -10 minutes to let her poop/eat/drink one more time...after all, the eggs won't get cold, nor dry out! If you don't do that, you could mist her head & saddle with a water bottle a few times, and dunk her beak in a small dish of water, to keep cool. Don't move her nest anywhere at this point, except out of the sun.
If nothing hatches by Day 23, and in this summer heat, remove her from the nest box, don't let her in, and put her in a wire cage or cool p[lace somewhere to break her, just because it could do her damage to stay in one place not drinking or eating for so long in summer.
Of course, I'm speaking from experience with my one broody, who hatched out two a couple weeks ago... her one egg didn't hatch, so on day 23 I quick ran to a neighbor for her pipped eggs, btw
She'll accept a 1-3-day-old chick put under her at night, if her egg doesn't hatch, also.
Also, regarding the heat, look in on her frequently, and if you never see he panting, she's likely not overheated, because she's not been eating much and is very still, so her metabolism is much slower.
A laying hen is frequently hot over 85F, because the egg trapped inside her before laying causes more heat to accumulate in her body. Her metabolism is fairly fast when laying, too, in order to digest & turn nutrients into eggs.
My mama hen was not panting this week, 95 & humid for days now, but my layers were.