Wikipedia has an entry giving some history and current status of federal election debates. If you scroll down, you will find an entry regarding the 2015 debates specifically. I hadn't previously heard of the court challenges regarding attendance to previous debates. If you have some time, it is worth a read. The only point that I am unsure on was regarding the "Up for Debate" campaign. The article appears to indicate that only the NDP and Greens have been invited, but I believe they were also hoping all Parties would attend. "Up for Debate" also has a petition running on this website. Here is a link to their website: http://upfordebate.ca/