Put your files directly in the trunk/ directory of your repository. Whenever you release a new version, tag that release by copying the current trunk revision to a new subdirectory of the tags/ directory.
Make sure you update trunk/readme.txt to reflect the new stable tag.
trunk/?You can, but it's silly. The .zip file the WordPress.org Plugin Directory creates will automatically wrap all your files inside a directory, so there is no need to put your files into a subdirectory.
If you have complicated plugin with lots of files, you can of course organize them into subdirectories, but the readme file and the root plugin file should go straight into trunk/.
Your subversion tags should look like version numbers. Specifically, they should only contain numbers and periods. 2.8.4 is a good lookin' tag, my neato releaso is a bad lookin' tag. Inflexible? Yup. Easy to handle and sanitize? You bet!
Note that we're talking about subversion tags here, not readme.txt tags. Those can obviously be any words you like.
Yes, by specifying the Stable Tag field in your trunk directory's readme file.
From the information you specify in the plugin file and in the readme file, and from the subversion repository itself.
Author field from the plugin header and Contributors field from the readme file.Version field from the plugin header.Tags field from the readme file.Plugin Name from the readme file falling back on the Plugin Name specified in the plugin header.Author URI and Plugin URI fields of the plugin header.trunk/ or the stable tag directory) after a version number change.The WordPress.org Plugin Directory updates every fifteen minutes.