Ok, then it's the approximation you don't like? I understand it's not for everyone
I think it's call rest energy yes
Depends what you mean by gain energy, in my mind a proton is already some energy so of course destroying a proton won't give you more energy. But if you mean liberating energy as photons, gluons and kinetic energy then you indeed have no transition that goes proton-> nothing with any mass or the like BUT you have the transition neutron->proton+electron+electronic neutrino+light which can spontaneously happen (it generally doesn't happen in matter because the increasing the number of proton increase the colomb repulsion and therefore decrease the mass gap, which can be enough for the right part to be more massive than the left one) depending on what exactly you mean by "destroying neutrons and gaining energy for that" it can count
Also you have the neutral pions, particles made from one up quark an one up anti-quark that can spontaneously transform to light or gluons and yet are the reason nuclei are can be stable
Yes
Yes, as the rule of tumb is "1g of energy ~1 Nagasaki"
We have WoBs implying there's nothing preventing a Soulcaster from transforming things into plutonium, which would create explosions far worse than 1g of energy