Well.... I have viewed your video a couple times. Your video did a very good job of explaining your concept, how you want it to work and what it is doing.
Normal iframe behavior: The web page loads and iframe will always start with the same content loaded, whatever is contained in the iframe tags. <iframe src=___>Originally you had all buttons and menu links on index.html load content into the same iframe on a 3 panel layout.
With buttons on either side of the iframe you were contrained to fit wider content into a small space down the center.
Your new concept...load wider content (external links) into a second iframe instead, on near-duplicate home page index2.html without right border, using a 2 panel layout.
The remaining buttons on the wider page you want to have open in the ORIGINAL index.html page iframe (assigning target to an iframe on a different page)
NOPE. Not gonna happen in any language. Contrary to the basic iframe behavior.
Every time index.html loads, you will get the same starting page for that iframe. Period.
If both iframes were on the same page, you could control behavior of one from the other. But not from separate pages.
You MIGHT consider either of the following before going ahead with your concept: Though attactive, your design uses an iframe to create a FRAMED website. Most people think of framed pages as static top and side borders while page body is scrollable. Search engines generally do not like them. Generally, the reason is that your main page will have no content. When the code is scanned, and spiders walk your site, your home page will have graphics and links and no related content. By forcing all your main content into the iframe, search engines will not rank you as well as a non-framed page. For example: Your goal, your history and other pages have the content. And you certainly do not want to have Google or Yahoo, etc send visitors directly to those "content only" pages! You expect your visitors to sit on the home page replace the body of the page. Google will say that your home page has no content. And since you do not want the content pages indexed by Google, what good is your website? Yes you can use Javascript to force your goal and history,etc pages in the iframe everytime, but still, redirecting incoming links is not good practice. Lowered search results. (Only one program consistently beat the problem by building in the iframe content into the home page also, CSB and it is no longer for sale. Can read bout the code difference here:
http://www.samisite.com/publish/spidersandrobots.htm#frames) Iframes are terrific for some things but I would not recommend you create a fully framed website around an iframe.
Having said that, it is YOUR design and if you choose to implement that concept then I would recommend you consider:
1) Best option: Make index.html a 2 panel page, eliminating a need for index2.html. Now all links/buttons can open in the existing iframe "iframebody"
Incorporate the right side elements below the iframe instead, among other options...
Include code that those content pages are forced to open within the iframe or the visitor is bounced to the index.html page. (Keeps the incoming visitor from loading a plain white page with no links, no site id.)
2) OR Change the name of the iframe on index2.html to the same as index.html "iframebody" and make all the links/buttons on index2.html load identically to those on index.html, into the target "iframebody"on that same page. So once the visitor is sent to that page, they remain there unless they hit home and it takes them back to the original page.
You asked if you could display the "content page" in smaller format (shrink it) to fit within your 3 panel layout, preventing the lower scroll bar.
NOPE. Iframes are simply a hole cut into your page allowing another page to show through (like a hole cut in paper and laid on top of a magazine. You do not change the magazine, only allow it to be seen through the hole in the paper.
The only way to clear that lower scroll bar is to either make the content page less wide in it's page layout, and/or widening the iframe to fit. Either way, it is more designing. (As you have already noted, turning off scroll bars is not the best option. Might work for some applications, but most vistors will not have the exact monitor/resolution/text configuration that you are using for testing and will run into issues seeing the content and get frustrated.)
I am sorry to dash your hopes on this one. It is obvious that you have spent a lot of time and effort on the site and I give you kudos for the work you have done! Very nice.