My experience has been that the "one-click" install will fail, reporting the the destination directory must be entirely empty for the install to take place.
Sure! I've done it many times. The trick is to install Joomla! yourself instead of doing the "one-click" install. .
Generally it should, though there are a few potential "gotcha"s. For example, you will have issues if any of the subdirectories below webroot have the same name as those in the Joomla! install, and you may need to "tweak" your Joomla! .htaccess file to make sure re-write rules don't prevent you from accessing those other subdirectories.
From your .ps. remark, you are likely to be "ok" as long as you avvount for the issues I mentioned above. Good Luck!