1) What data confirm that the rate of stellar black hole formation is increasing?
2) Antimatter (positrons, antiprotons etc.) does not have negative mass it has positive mass. It is distinguished from normal matter by having the opposite charge (and some other esoteric properties).
3) The bit about antimatter traveling backwards in time appears to be the Feynman–Stueckelberg interpretation. Now these were seriously clever guys, but really its a mathematical formalism rather than a physical reality. But if thinks travel negatively in time this does not mean things happen instantly with not time at all, they take the same amount of time but in the opposite direction.
4) A consequence of the Feynman–Stueckelberg interpretation would be that the gravitational force between matter and antimatter would be repulsive. This would be difficult to test, Health and Safety legislation being what it is.