Wel if you haven't patched the exploit you can alway slog in using it. If you have patched simply restore the old session.class.php and use the exploit.
Yes as admin you can put anything you want in the entry even HTML. The script deals with any HTML tags and AGcode in a post when it is submitted.I assume you have access to your admin again? If you do then I recommmend looking in this forum for my fix for this exploit.
If you are using Advanced Guestbok 2.2 then you are at risk from an SQL Injection exploit. In this case you will find the fix at http://proxy2.de/forum/viewtopic.php?t=3650
by giving a file the .php extension you are telling the server to pass it to the PHP processor before sending it to the browser. The processor then looks for anything between <?php and ?>. The templates in the guestbook are turned into strings by the guestbook and so the templates are never sent to the processor. To put some php into the templates you will have to do the PHP in the script and have th results turned into a variable which you can then place in the template.
I believe the daily counter uses flat files. Do a search for silvercounter if you want a DB based one. I have used Silvercounter myself and it's very good.