Ok, so It's only the mass change that is involved.
Only what mass changed that is involved in what?
Nevertheless, there are only certain conditions where any measurable conversion is going to take place
True. As already noted, the mass change in chemical reactions is (as far as I know) too small to measure. That doesn't mean it doesn't happen. So how is it relevant?
Nuclear fission requires high speed neutrons
. Nuclear fusion requires extremely high temperature environment.
The mass change involved in energy flowing through medium would undetectable.
What does "energy flowing through medium" mean?
The mass change would be measurable if the energy change were large enough so, again, so waht?
Since the first one is measurable it is particularly interesting, but strange things happen at the atomic scale.
What strange things?
And how is it relevant?
In summary, you have said nothing relevant to mass-energy equivalence, so what is your point?