To remove the widget, you need to drag it to the top of the screen where it says "Remove" while long-pressing on it and holding your finger there, as if you're going to drag it somewhere but haven't made up your mind where yet. As for the "Ok Google" detection, I can't get it to work reliably, even with Bixby fully off. Frankly, it irritates the heck out of me. I think we just have to live with Bixby for now until someone figures out what the blue blazes is going on and devises a fix.
I figured out how to do it (finally). First, you need to fully deactivate Bixby by going to the Bixby Homepage, going to the upper right corner, diving into Settings, and clicking off all those switches so that she's completely off and not listening. Then, you go into Google Assistant and turn Voice Activation on, train your voice, click on the buttons to allow activation from any screen. If you're not using Google Now launcher, install that too. If this doesn't work in letting you summon Google, you need to go to the Apps menu, find Google, and then "remove updates", then re-update it. This will reset everything and worked in fixing my issues with trying to summon Google Assistant.