Tougher Organs wouldn't shift balance that much, they'd enter crit just as fast as they do rn, they'd just take longer to die while in crit on the ground. Don't worry too much about balance, and making humans take more damage would make them enter crit quickly and still die slower. (So it's not a good idea unfortunately)
The rest of the points I agree with, but please understand that Unathi, Tajara, Skrell, etc are already stronger than humans in almost all of these ways. Humans getting a simple increase in organ resistance or the like wouldn't shift balance.
Unathi's scales are represented already ingame by a brute resist.
Humans are only the baseline species, they aren't the baseline mechanics/statwise, as said previously... And they're only the baseline species because they're the only ones that don't require a whitelist. If Unathi, Tajara, or Skrell etc didn't require a whitelist, Humans wouldn't be the baseline species anymore.
In conclusion here, if Humans would need concessions to get a simple increase in organ resist etc, then unathi and tajara would need massive decreases across the board to balance, it doesn't add up.