Not even close to enough information to make a real decision on this, but...
- In most places, there must be an underlying trespass to support a burglary/home invasion charge. Deceased was a minor, and his residence is that of the custodial parent. Sounds like that was mom, so it was his home. No trespass, so no burglary/home invasion. (Yes, there are some very limited exceptions to this that we can't get into here without a lot more facts.)
- Interesting thought about COVID-19 and self defense. Feels like it wouldn't fly absent shooter really knowing or having reasonable cause to believe the kid was infected. And, I'm having some serious doubts about COVID-19 = delay weapon to justify a shooting.
- Yes, a parent/step-parent can use the legal defense of self-defense for defending against an attack by a minor. Some lawyers play the "he was a child" game when dealing with a physical attack by a minor. For a 16 year old, likely an almost full grown man, good luck getting that to fly.
I'll be watching this develop...