but most peers wont support an incorrect input (multiple incorrect inputs at same time would be equivalent to a 51% attack), also one incorrect input doesnt undermine the system because the xrate for coin release could be the average of 90 days for example, plus there is a maximum velocity or inflation.
The problem with tracking difficulty is that if you release coins according to difficulty you create an imblanced system where your encouraging more and more miners to mine without sufficient overall adoption of currency. I think there needs to be a balance between mining and broader use of the currency by non miners.