These brace electrodes have a so high resistance that using these it is impossible that the intensity is high enough to cause internal tissue damage. (maybe slight electrical burns near the electrodes but that's all)
If copper handholds have been used, and if they were wet, the intensity may be high enough to cause some internal tissue damage.
However i don't think the zapper is necessarily the cause of internal tissue damage, it can be some helminths moving from one part of the body having a high conductivity to one part of the body having a low conductivity (demystifying one more time your argument that the intensity or current flow is the same in all parts of the body) and this may cause irritations, cuts, scratches, pressure, obstructions.
Another possibility is if one person has a high load of conductive metal (lead, copper, tin, ...) in one part of the body, it can probably cause an increase conductivity in this part of the body (the toxic metals can be eliminated through the nails and the hairs)
To resume, i don't think the zapper is the cause of the damage, except if internal electrodes have been used inside the brain...