(1)I spent my 4th year Computing project on implementing time series forecasting for Java heap usage prediction using ARIMA, Holt Winters etc, so I might be in a good position to advise you on this.

Your best option by far is using the R language, you can call on the forecasting libraries provided by R, through Java by using the JRI library found here. R is well documented, free and open source. You can even run R on a server and then make calls to it via command line using Rserve, which then returns forecasts over HTTP but JRI is the local equivalent if memory serves me correctly.