I've come up with another idea.... "the quartz theorem"
The nether is queen griefs realm and quartz as in nether quartz has time keeping qualities as regular quartz, as that's how none digital clocks are made, so hypothetically the nether aspect of nether quartz could have effects on the space time continuum, and excuse my actuations here; "queen grief is bound to the nether". and as such would benefit from potential time altering effects from the nether quartz.
So if that previous statement is valid, then the effects of the quartz could be keeping her immortal, the possible alterations in time could be sending any possible damage or vulnerabilities to an infinite end never reachable future.
So... if that is true then some fort of quartz based magical weapon, could sever her link to it, and maybe render her mortal... and killable maybe?
if not that then make some sort of time portal, and go back in time to warn people about her before it happened?