Please don't tell me I don't understand, because I definitely do. You have files you want to protect from people who aren't logged in, and you only want them to be able to open the files and view it through the browser, not download it and pass it around. You probably also want some sort of URL munging so people can't directly link to the files as an extra level of protection.
What YOU do not seem to be understanding from me is that you can't easily make WordPress recognize the existence of a n-standard folder in the system. You can't just plop a directory in your public_html directory and expect WordPress to know what to do with it. .htaccess can't tell if someone is logged in, either, so using that won't work.
What you CAN do is use a plugin that will protect your files as you upload them through the media uploader. That's the simplest (and free) way to pull off what you want. I you want more customized capabilities, you'll have to use external scripts, or premium plugins, and/or hire a developer to customize it for you. not free. Those are your options.