Black Hole: mass density or energy density?

Question

Recently, I read a quora post in which the OP asked the following question: Can a black hole be formed from accelerating a body and increasing its relativistic mass to the level of a Schwarzschild black hole? This question had me confused because of an apparent conflict of principles:

1. On the one hand, a well received answer to the quora post suggests that formation of black holes are only dependent upon the invariant rest mass, instead of the relativistic mass. Therefore, acceleration of any body to arbitrary speed would not cause a black hole to form. (See: https://www.quora.com/Is-it-possible-to-accelerate-matter-so-much-that-it-becomes-a-black-hole-in-one-frame-of-reference-and-not-in-another)

2. On the other hand, a quick search on wikipedia indicates that kugelblitz (black hole formed from concentration of energy, possibly with zero rest mass) is theoretically possible.

So my questions are:

1. Are there theoretical models for how a kugelblitz could possibly form? (The wikipedia page mentioned Wheeler's geons. But I do not have access to his paper and cannot tell if geon solutions are actually black hole solutions in any sense)

2. If the answer to question 1 is yes, energy can indeed create black holes. So, from some reference frame A moving rapidly away from the earth (or really anything for that matter), will the earth have enough energy density to become a black hole in A?

Personally I believe that both answers should be no. But I'd love to hear from experts who know a lot more to judge.

Answer 1

A fast moving mass will not become a black hole. See the existing question If a 1kg mass was accelerated close to the speed of light would it turn into a black hole? for a discussion of this.

However suppose you take two fast moving 1kg masses moving in opposite directions and collide them. In the centre of mass frame you have the original rest masses, i.e. 2kg, but you have also concentrated all the kinetic energy into a small volume at the point of collision. So at the moment of collision we have everything at rest and a very high energy density. If you make the speeds of the 1kg masses high enough this will form a black hole.

I have to confess I don't know if this argument applies to light beams because unlike the masses a light beam has no rest frame. You cannot argue that the point in between two colliding light beams is a rest frame for the collision. However it seems plausible that by concentrating the light from multiple beams into a small volume you could create a black hole.