Organmonkey
Versed in the lewd.
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The manga does say Karin and Kushina have special dense chakra. The super dense chakra is also seen as a prerequisite for holding the nine tails, or at the very least that's why Kushina got it and why the woman before her got it.The Heal Bite as you call it was definitely something that Karin's mother had and it's overuse is canonically what killed her.
That said, I say it's a bloodline ability because I don't believe that her mother would teach her that ability if she could avoid doing so, especially as I don't think she had any illusions about what would happen to Karin if she died or became unable to use the ability.
This sounds a lot like fanon.
The rest is sort of best guess since it doesn't go into more detail than that and the in series noted ability for Uzamaki to be resilient.