Yes a conical horn will start cutting off appox. 20% higher in frequency than what the flare rate is. So you would need a larger horn to achive the same cutoff.

::I know that the horn has to be large enough to house the other speakers, but how much of it is needed to load the 1" driver?

I have no idea.

::And, if the coverage pattern were opened to 90 degrees, what might be expected for an off-axis response? Will it become beamy?

It would hold its pattern down the the cutoff of the horn but off-axis the higher freqencies would be more jagged in response and at a lower level. All horns start to beam at a higher frequency especially a conical horn (But I don't remember what the formula is). CD horns try to stretch the beaming out effect to the upper octaves where the ear's not a sensitive.