1. Using dash instead of underscore as a space replacement. Underscore is a much more natural as a space replacement and dash is actually a punctuation character. An article gives the following reason: "Text viewer applications (browsers, editors, etc.) often underline URIs to provide a visual cue that they are clickable. Depending on the application’s font, the underscore (_) character can either get partially obscured or completely hidden by this underlining.". It's not convincing at all. I've never encountered this glitch.
2. The keep-it-simple rule applies here. Although your inner-grammatician will tell you it's wrong to describe a single instance of a resource using a plural, the pragmatic answer is to keep the URI format consistent and always use a plural.
But why plural and not single? English is a weird language and it has numerous exceptions for plural form. Isn't it simpler to use single form? I'm always using single form everywhere, works fine for me.