The equivalent head of water to meet the 3 PSI minimum operating pressure is right at 7 ft. As long as you can hold that hopper head (or higher) it "should" stay open. That seems somewhat 'tenuous' though, if the majority of feedback comments are to be taken seriously. This appears to be one of those "You get what you pay for" kind of imported items, assembled perhaps by 12-year-old Chinese kids. I'd move on.
Depends. The specs state that it requires 3psi. That is equivalent to 7 feet of water. So, if you have over 7 feet (vertical) of water going through it, it should work.