That’s like asking if you can turn off <form>s. Not sure why you’d want to. It’s a perfectly reasonable component that doesn’t need to be abused for ads.
I use <dialog> a lot when building websites. I don't run any ads nor promotional content so if that element was blocked it would break lots of features.
Seems like you could, since it has an attribute and a pseudo class to select on.
document.querySelectorAll("[popover]:popover-open")
should select all visible popovers. AFAIK uBlock uses regular CSS selectors.
Nope. Not if the pop-up is first party.
Yes you can - uBlock Origin lets you apply CSS rules, and popovers can easily be hidden with the [popover] selector.