"The Center Channel/PA Column alignment like I built was designed to lie horizontally if it's for Center Channel duty."
there is no getting around the simple physics of off axis comb-filtering when a speaker like this is turned on its side. when the different distance between one driver and another playing the same frequency is 1/2 wavelenth, it will destructively interact and cause a big dip in frequency response similar to the response of a crossover network wired 180 degrees out of phase. if it doesn't matter to the listener, then great, but it is not accurate. if you can listen more or less on axis, then it will be fine, but might have floor/ceiling bounce that need to be dealt with just as uprights need to work with wall reflections.