“How to explain then that subatomic particles traveling at near the speed of light, decay much more slowly, compared to the rates at which they decay when at rest relative to us.”
I am not a physicist or astrophysicist, but those who are and who write arguments against the dilation of time theory believe that there again, gravity “relative to us” is affecting the rate of decay, not “time”. The affect is an “apparent” alteration of time only because we do not actually measure the energy/wave form we call “gravity”, though in many ways we claim to measure its affects - sometimes.
There is no difference in the gravity. Both the "at rest" (Meaning not moving anywhere near the speed of light), and the "high fraction of "c" (like 99.99%) particles are in the same gravity field. That of the earth. The only difference is the relative motion.