I have used mission couplings and air tested the lines to 5psi before. Sewer lines, and house services. These were 6" - 12" and when you put air to them, they would blow up like a balloon, but they never failed. Usually we would try to have them buried though when we did it. Sometimes the only way to tell if you solved the leak was to go about it like that.
With that said, you are not really creating any pressure like that are you? It is a suction, and the return should not have much resistance. But, there are couplings called no-shear ones, and they have in addition to the two hose clamps, a stainless steel sleeve around it.
I think it would work, now would I do it, no. I would use a union, whatever the benefit of using a mission coupling would be, it would matched and exceeded by a union.
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