Well there are a few things that have not been mentioned in these theories. First of all, Jack Harkness is not human so we can not expect him to be the same as us biologically. The fact that there is a reference to the Face of Boe being pregnant in an episode actually lines up what we know about Captain Jack. In the very first episode of Torchwood he says:
"Contraceptives in the rain. I love this planet. Still at least I won't get pregnant, never doing that again!"
Jack Harkness having children (as the father) is also a part of the Torchwood, Children of Earth storyline. Those children are mortal. Just because the universe will not let him die does not mean that is the same for his children. Also, by the time of the episode, "The Last of the Time Lords" Jack has already lived on Earth for over 100 years and there have been references to him being stuck in a time loop while he was with the Time Agency (in the episode Kiss, Kiss, Bang, Bang) so there is no telling how old he was even before Rose made him a fixed point in time. Add another 5 billion years to that and there is no telling how his body would've reacted to all of his space travels and encounters.
Also "The Last of the Time Lords" episode clearly shows that Jack does not immediately recover from injury, only death.