Mass and energy is not equivalent, they are different aspect of either mass or energy, = means equivalent and M = E is not the equation.
There is an amount of energy that you can get out of an amount of mass, that is not the same as saying mass and energy is equivalent.
You just cant interchange the two different things.
E = MC2 is correct: However M = EC2 is incorrect.
M = E/C2 and E = MC2
E, Energy do only go at the speed of light: M, Mass can never go at the speed of light. That is not an equivalence.
Can energy or matter exist without spacetime? C is space time, as c is a speed or velocity a length of space over a length of time.
You can't have E or M or a relationship between the two without spacetime.
If spacetime was zero, energy would be infinite... Any amount of energy expressed over zero space length and zero time length is infinite.
If spacetime length was zero, there is no length to contain matter with mass.
You need space to put things, you need time for those things to exist over space, energy is not a 'thing', it requires no space you can pack any amount of energy into a single point of space, you can pack exactly 1 amount of matter in a single points of space.
Or "you can't have dynamics without time"