I made a mockup skeleton of what I think you want. All the ugly colours are just examples, you can delete/change them as you want. But it is a good idea to put in some colours while working, then it is easier to see where you are and what is happening.
The menu can also be edited and styled. And when styling h1 through h6 they don't get the 'class dot'.
The secret about avoiding scrollbars etc on the included files is setting their dimensions so that they fit into the 'holes' in the main document, including margins, paddings and all. And also, the middle part, the 'main' div of the document is not supposed to be included. I think that's where you got the problems.
Note the start of each html document. You have to include the doctype and the charset, which I have done.
You will see that the included files have embedded styles. There is only one of each, and they are all different, so I thought that to be the easiest.
Further it is good practice to write all the code in lowercase. You must be having a very aged css editor, I think it's more than 10-12 years ago that they used uppercase. It's also a good ides to have all the file names in lowercase. Everybody is so used to it now, that they might be getting trouble if they want to address one particular page. Unix servers, and they seem to be the most common ones, don't know that A and a are two instances of the same letter, they are regarded as two different ones.
Just let us know if you need more help
Inger, Norway
My work in progress:
Components for Site Designer and the HTML Editor: https://mock-up.coffeecup.com