Which roku? Did you check Hulu & Roku sites FAQ, & your seller?

AFAIK Hulu comes pre-installed.

Disclaimer: I'm not entirely familiar with the Roku system. But I reckon it should have some type of "app store"? Allowing you to search for and install the Hulu app, if it isn't pre-installed in the first place.. Which I also think it is..

For the LG smart TV and DNS.. You should check the TV's network settings for the option to set the DNS it should use in commucating. Setting it to that of USA (and if working..aka, not blocked by Hulu), should allow you to not only download the Hulu app onto the TV itself, but to also access USA content and watch without the need of an external device... Works on my Samsung TV... Same concept applies to Netflix.
Yeah, as suggested by dwRK.. The mobile version of the YouTube app does require a mouse to simulate the touching of the screen. There's however, a more TV Box friendly YouTube app (please Google). I can't recall the exact version.. But it's either the one made for the Android TV OS or its based off it. With this version, you can navigate the menus and videos using the d-pad of your remote.