Matt Stassler is always worth a read.

Time's short, so I shall probably have to take it a bit at a time. Couple of questions from the first one to start with.

"Mass Energy of Particle 1 = Mass Energy of Particle 2 + Motion Energy of Particle 2"

Does the Motion Energy of particle 2 come from the action of decay?

"Since motion energy is positive, particle 2 must have mass energy less than or equal to the mass energy of particle 1."

“Less than” seems OK, but how could energy/momentum be conserved if mass energy of 1 = mass energy of 2, but only 2 has motion energy?

I suspect there will be lots more questions, but let’s get these out of the way

There never was nothing.