Since we’re sharing theories…
What if the Doctor does kill Davros to bring Clara back (as well as the TARDIS)? Then they go off & have a whole bunch of adventures until the Doctor realizes how big of a paradox getting rid of Davros & the Daleks has created. Therefore, in order to fix the paradox, the Doctor has to ensure that Davros lives…at the cost of Clara’s life…
Of course, that would mean the end of Missy too, & I don’t want to see Michelle Gomez leave yet…
Then again, “Death is for other people.” 😉