I found that the myco fungus act as an intermediary in nutrient intake of plant (food in exchange of photoperiod process products) and improve growth, also presence high level of nitrogen in substrate improves fungus colonization, so answering on first part of my question "can the myco fungi decrease risks of nutrient overdose in rich organic soil?"- I suppose yes it can. Interesting moment, if soil is poor - presence of mycorrhizae fungus can slow plant's growth due to contention for nitrogen and parasitize the plant.
Answering on second part - I partially agree with you. If soil really hot and nitrogen level extremely high - there's nothing to do, only transplanting plants into safe substrate can save them. But you can add mycorrhizae before or after transplanting into safe soil, thereby help your plant survive due to anti-stress and root growth stimulant properties.
At last, that there's many many many factors influences on symbiosis, so all aforesaid has no practical utility for the ordinary grower like me. Conclusion - know your soil and all be ok.
Offtop. Also I learned that the better way to receive really living myco fungi it's walk to forest and take compost mass under fallen rotten leaves near healthy wood and then add to your soil mix. Thereby you'll receive a native myco-preparate adapted to your climate conditions and not foreign with an unknown composition with unknown consequences for biocenosis in your location. That's really useful