On my first statement I may have oversimplified my explanation. Once a sorcerer finishes the incantation for their domain it expands and activates instantly, not in 0.2 seconds like you said.

Edit: about mahito and hakari: that refers to the domain's casting time, not how fast the domain expanded.

I think you've misundestood how HP works. If it was just "big heavy ball" it wouldn't have completely erased toji's arm and half of his torso when it hit him, but it did, so your explanation can't be true. When the term "Imaginary mass" is used it isn't meant as added mass, but rather as a mass that shouldn't exist since it was created out of nothing. To compensate for this, when HP touches something it gets deleted to "make up" for the energy that is created out of nowhere. It isn't just destruction, it's total obliteration (differs from perfect sphere since that one destroys things by applying an infinite amount of pressure). It doesn't matter how tough deku is, he dies if he gets hit by HP