Is there a way to upload custom layout to a test page so that it doesn't affect the primary site while working on it. Just in case you do something really bone headed?

We have 500 people in our guild, and don't think for a second that I will ever hear the end of it if they cant access the forums for 10 min.

testing of custom layout is built in. Just hit the Test button before you hit Go Live.

Ok, what happens if I believe that a layout is what I want, and then when I go and apply it, it breaks something fundamental? Is there a way to revert to the default layout?

Yes if you go back to the advanced layout page. Where you click to download the layout there is an option to download previous layouts. It does make backing out very handy.

There is a history link. When you click this it will show all the files you uploaded and the date stamp of when. You can download one of these and re-upload it to restore your site to before the changes.