How is he surviving the execution though kaido would have known hes alive.
Not necessarily.... one simple interpretation which would require Oda to showcase this between Oden/Toki during prison dialog is Oden who told Toki that if he should die even after surviving the execution of boiling, he wants to use her powers... that's literally what Oden stated...
What did they plan to make him survive? how did Toki manage to fool everyone with her ability? not sure how...
BUT a very bold move by Oda would be that Toki is there hiding from everyone in execution plaza.... and after Oden survived the boiling... and orochi and kaido decided to betray their words, MAYBE, it's possible that Toki transported the bullet into the future, while Oden pretended to stay dead...
Kinda like Kakashi using Kamui with bullet from Pain
Hell, maybe Oden himself moved his head so fast despite his injury where the bullet barely grazed him but pretended to be DEAD... and after everyone thought he is dead, he went to Toki and they traveled in time...
Exmaple of that is Abdul from Jojo bizzzare adventure surviving that bullet and pretending to be dead
Maybe some different explaination even... point is... I think it's really just a simple answer that he survived the execution and traveled through time to be here....