Can i Still return the HTC 11 to HTC.com even if I opened it up and tried it?

I'm sure you'd be able to! It sounds like you're well within your return window.
 
Why return? Don't like it?

I'm probably returning mine because there aren't any accessories for it. It's infuriating because I love it. It's fantastic. But there are a handful of crappy cases and very few screen protectors. There is no way to protect the device and little repair support in the US.

Pisses me off.