Well, I'm not sure what the problem is. The attached files work for me.
There is no problem when including amsdefs.inc, as you have done in the layout file that you attached. The problem is when I replace this with "Input amsmaths-seq.inc" in the layout file, so that I may get sequentially numbered theorems and such.
Could you try replacing "Input amsdefs.inc" with "Input amsmaths-seq.inc" in the very same layout file that you had attached, and see if you get the error saying \theoremstyle is not defined?
Thanks a lot for your patience and help. -- Philip