Question: Can 9-speed cassette be used with 10-speed chain/derailleur or vice versa

True, I kinda did hijack it. But, I was really asking more questions for clarification for the OP’s benefit. Yea, that’s it!

I think he can still do it as he doesn’t even have the 10-sp bike yet. He just needs to make sure that it has DA bar end shifters (vs. Campy or SRAM).