Sorry to hear you're having issues, but I'm sure we can get you fixed up.
First off, I'm assuming you meant you're using Joomla 1.5.23 (not 1.5.3). Also, what "CB Integration fix" did you make? You shouldn't need to modify things to work with CB, so let us know if you did, as that may be part of the problem. The CB Profile Plugin should do everything you need, hopefully.
As for the case of the hidden comments/login button, it's likely that you have the System - Cache plugin enabled. If so, please move it to the last position (highest number). That plugin caches the entire contents of the page, and if it does so before the System - JFBConnect plugin can do it's work, that will be a problem. The other alternative is that you have another Facebook extension installed. If you open the HTML of your page, and search for 'all.js', it should come up only once. If not, that's bad. If you want us to take a look at the front-end of your site, just post (or Private Message) the URL.. we can usually diagnose these problems quickly.
For the 403 problem, we'll definitely need to see the page to figure out what's wrong. That's not an issue we've heard of, but sure we can come up with a solution.
Finally, we just released JFBConnect 4.0, which has quite a few improvements and other fixes. Don't think it will fix the above problems necessarily, but if you just started with JFBConnect, you may want to upgrade to the latest
Thanks,
Alex