Hi, and Welcome to MODx!
I don’t know about HTML5, I don’t use it yet because many things are not supported by browsers, anyway I’ll try to answer the other questions:
This has me really excited, but what if I wanted my right bar to be slightly different for each page?
I think that there are several way of doing it (due to the flexibility of MODx

), here are two examples:
1) Using Template Variables
http://wiki.modxcms.com/index.php/Create_TV-Based_Chunks
2) Using IF Statements with PHx
http://wiki.modxcms.com/index.php/PHx
I am pretty sure that there are other ways but I am still a noob

.
1. Can their be two content sections on a single page?
Sure, you can have one, two, three billion content sections. You can add a content section in this way:
1) Go to Elements > Manage Elements > Template Variables > New Template Variable. Fill in the fields like this:
Template Variable Name: content-2
Description: Secondary content area
Input Type: RichText
Template Access: Select the template where you want the additional content area to appear
2) Open the template and add the Template Variable like so: [*content-2*]
3) Open the Resource, you should now see the new content area at the bottom of the page.
Hope this helps