Yes, I think infix and export would only be used infrequently. For export it’s “I really wish this could be an enum, but it needs to stay an open sum”. I am also fine with delaying export and restring this initially to enums only, until we see more convincing usage examples.
I know that the infix change did upset some people’s well-reasoned conventions. The problem was that the code I saw in the wild did not have clear and uniform conventions when to use infix, so the code was typically all over the place. For intersect and symmetric operations like it, there’s always the possibility to write them infix in backquotes. Did you try that? For me personally, I have found them to work well enough.
No, export as a clause does not allow any modifiers.