Okay so here goes. Yes I'm driving, if you count the fact that I'm waiting on the flag man for a one-lane section of Road under construction as "driving". Don't text and drive, don't be like me. Also I am using speech to text, so please forgive the lack of appropriate punctuation.
If I understand you correctly, you have one year old winter hearty breeds which have recently stopped laying. You are in Canada and have been experiencing unusually hot weather, 40 degrees Celsius oh, about 105 Fahrenheit. You have also recently changed feed, from a low protein layer mixed with treats to an all seed mix, and you are inspecting for mites and other parasites today.
So here goes.
You absolutely could have birds at start of molt. My eldest birds are from about that same time period, and a couple are starting to molt. I have a couple winter hardy breeds, including Dark Brahma, at 95 and 95% humidity. It's hard on a bird. A significant number of other posters on BYC have mentioned early molts this year, unsure as to cause. But an inspection of feather condition and an observation around the hen house for an excess of shed feathers should give good idea of whether or not they are currently molting.
Your mite inspection may best be performed after dark with a flashlight while the birds are on the roost. Bring a white paper towel and wipe the roost down, you are looking for red streaks. That will maximize your chances of finding both mites on the birds and mites in the house, which will cover most of the species.
My real concern, however, is what you are feeding the birds. Particularly under stress conditions, you need a commercially complete feed and there is no need for treats at all. If you do feed treats, they should not exceed 10% of the daily diet by weight.
16% protein is the bare minimum. With the treat you were offering, their protein ration may have been as low as 12 or 13%. The amino acid content of their meal was probably low in lysine as well - none of the treats you mentioned are high in it, its almost or completely absent in corn and most grains. Recommend an 18 or 20% complete feed with oyster shell on the side. Higher protein feed will help them speed through molt as well.
Your birds have not stopped laying due to a lack of calcium. Feeding extra calcium won't help, and if it is mixed into their food so they can't sort it out, it can harm the birds long-term. If your birds were short calcium, you should have noticed weak shelled eggs and even shell free eggs possibly before they stopped laying. Since there was no mention, but you provided such a thorough listing of other symptoms and recent changes, I suspect that is not the case.
Anyhow, that's my thinking. Looks like I'm finally going to get to use this one lane road, I will check back in later. Good luck with your girls!