Then, in contrast, the narrower the horizontal coverage, the higher the HF cutoff (taking cone breakup modes into account). Am I on the right track? And, I'm assuming that pairs of like drivers sharing the same horn would be subject to the usual coupling rules when baffle-mounted, right?

I'm trying to be careful with making intuitive assumptions about how a cone driver would work in a horn as opposed to a simple 4th or 2nd order box, hence all the Q's.