With respect to Born probabilities, TI is on the level of MWI, it has no derivation for them. Similarly, its ontology is rhetorical rather than rigorous.
A central issue for any zigzag-in-time or retrocausal theory of QM would be vacuum polarization, which was the stumbling block for the most serious effort, by Feynman and Wheeler. But Feynman-Wheeler theory is also where the path integral was born, so TI advocates could say, we just need to go back and finish it properly.
With respect to Born probabilities, TI is on the level of MWI, it has no derivation for them. Similarly, its ontology is rhetorical rather than rigorous.
A central issue for any zigzag-in-time or retrocausal theory of QM would be vacuum polarization, which was the stumbling block for the most serious effort, by Feynman and Wheeler. But Feynman-Wheeler theory is also where the path integral was born, so TI advocates could say, we just need to go back and finish it properly.