Count:

"But wouldn't you agree that Chen tai chi is more of a sideways fighting system than say, Hsing-I?"

I dont know how to answer that because I am not really well versed enough in the workings of Xingyiquan to compare. Further, Taijiquan that I trained has sideways, straight, round, and all kinds of others methods of approach/attack. It depends on skill and preference of course.

"...Or are you agreeing that centerline concept is more complex in bagua, Hsing-I or tai chi, than just a 2 dimensional line that runs up and down your opponents body?"

Again, hard to compare without enough knowledge, I was not clear that this was the limit of the cneterline idea in something like wingchun?? If so, well yes Taijiquan centerline ideology is very complex. Basically Taijiquan, you guard, change, manipulate, fake, and attack many different levels, and "dimension" centerline. its plain physics really regarding centerline. I dont know that "centerline systems" are all that simple, but if they are, the simple ones are quite important to master as well in Taijiquan. This is why, regardless of which version of centerline we discuss, in my experience Taijiquan is still a very obvious and intended "centerline system".