Those aren't editable, at least currently. The logic is that most commands, and all shell extension items, would not work as expected on FTP files so they're limited to a handful of things.
I'm not sure that would work with FTP sites. Even just going to an FTP URL by pasting it into the address bar is slightly iffy, unless the site uses anonymous logins or the URL includes the user name and password.
To the end, or after the domain part? It should be there after the domain part, to indicate the root folder. (Without it, at least some parts of Opus would interpret it as being relative to the site's default folder, which often isn't the root.)
Adding quotes around paths in the path field doesn't work for any type of path. (Quotes will be stripped out if you Ctrl-V paste into the path field, which happens before you push return. But if you add the quotes manually, it'll mess things up. There's no reason to ever do that.)
Please send us the crash logs.
Could be an issue with the server not responding the way Opus expects. Would be better as a separate thread, with details of the server type (so we can try to reproduce it) and what the logs showed (so we have more idea of what's happening).