I do feel your pain and like you my biopsy wasn't positive for Grover's, yet I obviously have it. I meet 4 of the 5 commonalities (including a blood cancer, leukemia which I no longer have). Good info and photos on dermnetnz.org which lists not only the more common forms of eczema, like my ACD, but rare forms like Grover's. . My derm told me for the test to show a positive the breakout stage must be at the right stage and my breakout was waning when biopsied.
My first break out (one year) was the worst, for the next 2 years about 6 months on and 6 months off, but each break out has been less severe. Now 6 months clear I am developing minor breakout on abdomen and started applying the Calciptreine Cream on all torso areas after showering and nothing has progressed in weeks. My pain had been so terrible with the initial torso, front and back, breakout I couldn't lean back in a chair. Forced to drive a car hunched over the steering wheel. Fortunately my sides have never broken out so I can position myself in bed and have trained myself not to move. Druing the day the pain is the worst and we all know about itching more obvious at night as our histamines are elevated in the evenings.
Steroids did nothing, also I tested positive to an allergy, I have (ACD) to one of the 5 classes of steroids, which carries over to 2 others, so that leaves me only 2 classes I can use.
The most helpful treatment I have found is Calcipotreine Cream, a vitamin D topical. Also if the papuales break and scab infection is possible and I use Clindamycin Phosphate, a topical anti-bacterial fluid applies topically.
Heat and sweat and friction must be avoided, a challenge, but try to limit outdoor activities to mornings.
The bottom line seems to be it needs to run it's course. The collagen keeping our cells tight has weakened and our skin no longer protects us from iritants. Even now, without a back torso breakout can feel very tiny bumps sub dermal. These are the places just waiting to break out again.
Fortunetly my first break out was just a year as I have corresponding with one person whose first break out was 6 years.
Keep researching, kg7382 just suggested a new info site and will go to that to learn more. Always hopeful.