I guess your theory and a traditional christian theory just will never meld. I believe to understand the concept, it assist to understand that eternal and immortal are not the same thing.
Using your physics theory that time applies to everything, everything will die in time, then nothing is immortal, nor will ever be. Therefore God wouldn't be immortal.
But god exists outside of time as well as within it (from a christian POV) for he is omnipotent (which is why time does not apply). Immortality appeals only to time and death. Eternal doesn't because is exists before and after.
From a scientific POV, I can see your point, but time isnt important to eternal life, only to immortality.
Immortality is the opposite to mortality.
To be mortal is to be subject to death. God did die. Therefore he cannot be immortal. (But was incorruptable).
And if you believe that everything is god - then that diamond still cannot be immortal, but always eternal.
Peace and Rapture