you seem to be heading in the right direction.
just a minor point, on your second sim, s2 is the cross sectional area in the front of the driver, so s1-s2 can't be less than 1/2 the driver width.
what made the lms-r horn build relatively easy was that the s1-s2 length and s3-s4 length were the same and the expansion was pretty much constant. that can make things a lot easier, but the result isn't always good.
he wanted a giant horn and that driver just happened to work in it. sometimes you get a little lucky.