Sodium chlorite is activated when its PH is lowered. Ingesting sodium chlorite will lower its PH as it comes into contact with saliva, mucous, and stomach content. While this has not been formally studied, it stands to reason that the sodium chlorite would be activated inside the body.
Chlorine dioxide is quickly broken down to chlorite ions. Studies with animals has shown that chlorite ions are absorbed into the blood and organs and eliminated through urine. It is speculated that something similar happens in humans.
The half life information was done, as you mentioned, by marking the chlorine atoms.
This is an excellent reference on how chlorine dioxide and chlorite break down inside the body.