what would be the maximum load that the world could carry in travling through space at the normal speed?

The Earth is part of a system that is balanced.

If the moon were more massive, it would have a different effect on the Earth, and vice versa. Same with our gravitational interaction with the Sun... our position is a function of mass and velocity, not unlike any other satellite.

Centripetal force is mv2/r, so if you change the mass, then something else will surely be affected.

It is not correct to say that the mass of the Earth does not matter and can change by several multiples with no ill effect. Common sense, alone, dictates that to be a bizarre statement.

The Earth is actually getting lighter, not heavier. But any small mass we add will be negligible since all things here are just being converted to other things.

Adding mass has almost no effect on the the speed nor the shape of Earth's orbit, up to several Earth mass, however crashing mass into Earth would have somewhat more effect. Neil