No. You can not use HTML tags in any field except body of the message. The maintainer of the script has the option of allowing or disallowing any HTML in the Message part of your posting. If they allow it, your html markup will appear in the posting. The common practice is to disallow HTML.
Start the line from space -- formatter will consider it as pre-formatted and will not touch it.
Your post most likely did not show up, because your browser did not reload the page, it simply pulled it out of cache. Please reload your browser and it should then appear. If you are board administrator set $post_redir to 1 in the configuration to force the reload.
Visit http://www.amsoft.ru/ruboard/ - the scripts are free for noncommercial usage.
Of course. Visit http://www.amsoft.ru/ruboard/samples.html; the very first Ruboard-powered web board is Russian Active Worlds Forum (be ready to speak Russian to visit).