Yep I reckon your spot on. The heat in the soil is from the microbial action I think bacteria are probably the cause of the heat but I could have it back to front, it’s the middle part of the composting process that actually heats up and if there’s enough volume will go thermophilic till the microbes in that stage have consumed all the available food and then it will start cooling of,and a more balanced population will start to take over. After that if you just add small amounts you can add almost any amendment and the biology will take care of it. I don’t add huge amounts of anything except gypsum into my base mix I do try to add a lot of different amendments that break down at different rates,then I almost continually add small amounts of stuff as a top dress and then fortnightly I’ll top dress with a good amount of well rotted compost or wormcastings to boost the microbe numbers.