Americaninlima,
I have installed and upgraded many versions of WordPress. One wonderful feature of WordPress 2.7 is the automatic update/upgrade features — that really work. I did the automatic upgrade from 2.7 to 2.7.1 as well as numerous plugin updates without any difficulty. I think you should be running 2.7, eventually.
I had the same feeling about the new admin area as you do — at first. However, I don’t even think about it anymore now that I have used it for awhile.
I must confess that when I upgraded my online blogs to 2.7, I thoroughly tested the new version first (see “Low-Stress WordPress Version 2.7 Upgrade“).
After all you have been through, if you have a backup for 2.6 that would restore your blog to perfect health, it might be a good idea. However, at the very least, backup the current mySQL database first so that you can retrieve any posts you have created since your last backup. If it were me, I would also backup all of the current WordPress files, too: this might save you some time when you try the upgrade again. I assume your old backup included both the database and the WordPress files? If only the database, you will have to re-edit and upload any theme files you changed.
With respect to Permalinks Moved Permanently, it might be worth a try, especially if you are thinking about restoring a previous WordPress version. What have you got to lose?
As a last note, and for the long term, you might think about setting up your own test blogs. I have a test bed on my local PC (see “WampServer: A Test Bed For WordPress“) and also have an online test blog living in its own domain that I use to test new versions and plugins before using them on my main blog. My main blog does not have nearly the content on your blog, but I would still not like to have to do it over again. So, I test and backup. This takes all of the worry and stress out of making changes to my main blog.
If you do restore to your older version, relax awhile and then try the upgrade again (possibly on your own test bed first).