Is there any canon support whether Dumbledore had a fallback plan in case that Harry fails?
As a start - Harry is hardly the best choice for the one to hunt down and kill Voldemort. There are more experienced people at hand who can do it and have much better chances to succeed.
Yes - Harry had a unique advantage of the double protection coming from his mother's blood that was later transferred to Voldemort. Yes - Harry is "the chosen one" according to the prophecy although prophecies depend a lot on what people do and are not guaranteed to come true just because they exist.
There are a lot of places where Harry could've been just killed together with Hermione and Ron and the whole knowledge about the hocruxes would've been lost.