C-17 landing in Chengdu

Is it clear which route they were taking, when transports the said remains?

I am not sure if you took the west route, all the couple dozens countries on the way would all give you a green light (on time); but if China gives you green light for the east route, it's pretty "staight forward" to fly to any nearby US airbase at west Pacific, thanks to the "Let's circle China" strategy, and all the way home.

Why Chengdu particularlly, is also one of the myth.

It is indeed strange... but like i said earlier... the sichuan earthquake relieve task was performed by 1 C-17, and clearly the photos shows 2 different planes from 2 different wing.