I have attached my solution, I ended up with more accurate results than the original solution. I'm guessing because I'm using the newer version of Alteryx which comes with an updated version of R and I'm assuming updates to underlying default assumptions.
When I complete all of the challenges I'll come back to this one and do it in the R Prophet package and see how the results differ.
Great forecasting exercise!
I'm not able to replicate the exact same result and the ARIMA-tool returns an error as one can see on the snapshot in the spoiler-section. I suspect that the ARIMA and forecast implementation has been changed in a later version. However I managed to hit the forecast correctly down to the 11th decimal. I conclude that it is close enough. The confidence intervals, however fluctuates +-1 unit(s). I tried to download the official solution and that has the same issue.
Slightly different values to the output file given in the start file. Downloaded and ran the solution workflow and came out with the same values as I did. Different values most likely due to a difference in the versions of the Arima tool being used.