I can help you with one of those questions. Java fern *can* attach itself without being tied down. However, you'd have to consider that the water current might hinder that, as well as you swishing the water around any time you're planting or trimming plants. The plant won't be able to attach itself if it is constantly being moved around.
I've read good things about using staples if you want to avoid the unsightly string. I just planted something else to hide the string, personally.