Can I put html tags anywhere in my posts?
No. You can not use HTML tags in any field except body of the message. Your html markup will appear in the posting, but some potentially malicious types of HTML may be disallowed and not appear properly. It can't hurt to try something if you want to.
It is good form to include portions of a message to which you are posting a followup. You can delete the unwanted portion (or whole thing) by "drag-selecting" the text with the mouse and hitting the Delete key, holding the SHIFT key down and using the cursor arrows to select text and hitting the Delete key or just using the Backspace and Delete keys.
What are the numbers at the end of the subjects in the index?
The numbers in parentheses after the message subjects represent the number of followup messages to that post.
If I fill in my email address will the spammers harvest it and spam me?
Why is " -AT- " in the email addresses instead of the @ symbol?
Your email address will be "masked" by changing the @ symbol to " -AT- " which prevents the spammer's harvesting spiders from recognizing it. It means anyone who clicks on your name to email you will have to change the " -AT- " back to an @ symbol in their mailer, but this is a small price to pay to keep your address away from the spammers.
Your post most likely did not show up because your browser did not actually reload the page, it simply pulled it out of cache. Please reload your browser and it should then appear. Some ISP's use poorly configured caching proxy servers between you and the web which can cause old pages to appear until their cache expires. There isn't much one can do about that except to complain to that ISP.