Ignoring what you signed and what you now think you are entitled to, you joined a company as a temp, covering someones maternity leave...you knew they'd come back and return to their old job...

Without being too rude you sound like you are just trying to make money from a company that is trying to cut back on costs. There isn't much difference in the mentality of these 'whiplash' ,'where there's blame there's a claim' schemes and what you are attempting to do. Get another job and move on.