I too have found most doctors have never heard of Grover’s. However my dermatologist was quick to run 2 biopsies and give me the diagnosis. But this group is most educated on subject. I told my dermatologist about cilantro and he was surprised at how it worked. But Grover’s has kept me from training and competing (running-Masters Track) since January. I have just resumed running. It was longest hiatus in 43 years. Still taking cilantro and maybe someday may get off the daily routine.

On this site we totally understand your misery and search for treatments or lifestyle changes to help.
As you have read cilantro, which I learned about on this site from a fellow blogger, thanks to Kimass1 and her Wed Feb 27th post, has not only helped me, but cleared me of 99% of my symptoms and the only treatment that helped. It sounds like you are experiencing your initial breakout. Normally the first breakout is the most severe and the longest lasting. I have read from one person who was dealing with a 6 year initial breakout, but I followed a more normal course of a 1 year break out. I was beginning year 4 with Grover's and a new breakout period so my rash symptoms were in the early stage when I started the daily cilantro smoothie. I noticed my breakout stalled in less than 2 weeks and I never got the extreme pain or any pain and only a mild initial itching that was gone within a month. Still the scabs held on for months. It began in the lower back and never advanced to the upper back and only had a few on my belly. I have never had it on my arms or legs, as this advanced form seems to be less common. Since you are severely broken out and in the initial outbreak stage it make take longer to notice any improvement, so don't despair and keep this cilantro smoothie going for at least 3 months. Now 5 months later since drinking my daily smoothie I still have a handful of scabs that fall off only to return, but no pain or itching. Most scab scars have faded. You read that most of use fresh, but frozen also has helped some. Tips on how to keep it a week are in our blogs.
You mentioned collagen, ours has separated and no longer holds our skin cells together in our Grover's regions. I believe this cellular restructuring is permanent and we no longer have our skin protecting us like it used to. Cilantro will be a temporary fix I believe, as our skin structural damage is permanent. That's why I plan to drink this daily for life. Right now you are trying any treatment you can, so it will be hard to tell if the collagen treatment is helping versus other treatments like cilantro, but you are doing what I have done and others desperately grasping for any help trying to get our normal lifestyle back and most importantly be pain and itch free. While your doctors were treating it incorrectly as heat rash, mine considered it a symptom of my initial form of eczema Allergic Contact Dermatitis, ACD. I would suggest the 5 Day Extended Patch Test for ACD, because I have both forms of eczema and they can have overlapping symptoms and you could too, but you can't have this 5 day test until your back is clear as the patches cover your entire back torso and need clear skin for a reading.
Most of us with Grover's were leading active outdoor lives like you. Have you gone to the dermnetnz.org site I have recommended and looked at Grover's site? It has a good simple list of supposed causes, for Grover's sun exposure is one, and also this site has very good photos. Even though my GD case is classic, my only biopsy came out negative, the derm said it is because the GD must be active to show up positive on the biopsy and when biopsied my breakout was beginning to clear up. They say it is not genetic, but my sister who was always told she had heat rash got a biopsy after I told her that I finally got a name for my torso rash other than ACD, she got biopsied and it was positive. Her symptoms have always been minor and only around her lower bra area. Even her itching is mild and never painful. My symptoms have been severe and like you restricted my outdoor activities and sleep.
Others like myself are sharing info about our cilantro treatment trying to help others. My next step will be to go to dermnetnz.org, contact them into looking into adding this information to their website.
I'm hoping cilantro helps you, seems it helps about 50% of us.

Welcome to this most outstanding group. I was so sleep deprived and miserable with my Grover's when I joined this group that I did everything suggested. My Grover's cleared up in just a matter of WEEKS. I hate the taste of cilantro so I make a VERY LARGE smoothie to mask the taste. Mine includes almond milk, strawberries, banana and blueberries. Almond milk because I am lactose intolerant. However, I took it a step further and went to wearing only pure cotton, sleeping in a cotton man's oversize T-shirt purchased on-line from a site mentioned in an earlier post and sleeping only on pure cotton sheets. I even began the Whole 30 diet and added chlorella for metal purging. I thought I would just stay on that eating plan a month but I lasted 6 months and just recently began introducing some things sporadically, if at all. I never got to the point of having scabs. I simply had a three dimensional intensely itching/painful rash that was diagnosed at first-look by my dermatologist, within a month of my having it. He said it is more common in Arizona because of the dry climate. I told him about the cilantro and he was very receptive. I have been using frozen cilantro the past few months for ease in preparation. I simply divide the bunches into sections and put in separate freezer bags. I do the same with sliced banana and my fruit is mostly frozen. I have even used peaches. Cilantro is also very good for the digestive system and I definitely noticed a change for the better. I tried the supplements when I could not make smoothies and they also held the Grover's at bay but I use fresh whenever I can. I don't want EVER to risk returning to a Grover flare up. We are all rooting for you and hope you will stay true to the cilantro, wear only cotton, and keep us posted.