2- conversely the price of BTC goes down from $14k and I do not put in any kind of stop loss, at 2x leverage I estimate my liquidation price would be roughly half of $14k so around $7k (maybe just over)... is this correct?
No, when trading on margin, the liquidation price, with x1 leverage, will come when bitcoin drops by 50%. If you use x2 leverage, then the liquidation price will be much higher than 50%.
3 - If I've understood both the above scenarios correctly, what is the benefit in using Spot or Margin? It seems to me that Derivatives does exactly the same as Spot and Margin with the added benefit of additional leverage if required! Is that not right?
I don’t really understand why you separate the concepts of margin and derivative, because they are one and the same. Derivative trading is margin trading, that is, trading using leverage and paying the funding rate every 8 hours.