Couple of answers from another (semi) old fart
1. Can you only use the beginning post in the thread? In other words can you reference a post farther into a thread or does it have to be the one that started the thread?
Each post has it's own unique URL, so you may link to any post in a thread. The easiest way to get the URL is to put your curser on the title of the post on the index page and right-click it. Then select "Open Link in New Window." You can then copy the URL from the address window.
Of course, like any post, it will show the entire thread at the bottom of the post.
2. I suppose there is some electronic wizardry in place so that someone can't make a post on the free pages that includes a link to a post on GC. Am I correct?
You can link to a post on GC (or BC, or the CBJN page) in a free page post. However, if someone clicking on the link is not already logged in as a GC member (or BC, etc.) on that computer, they will be asked for their password.
And of course, if they do not enter a valid username and password, they will then get the "You are not authorized . . . " screen instead of the link.
Simple, no?