I am looking to create a Mock implementation of the IEngine interface (actually merging is the only spot I am blocked at, the rest is pretty easy), but I am unsure of the exact way to calculate the various types of merges(same side orders, opposite side orders on both the winning and losing side).

Is there any object to handle the calculation of the outcome of a merge pre-built, or if not, is there any way I could get access to the way this is calculated so that I can accurately mock this?

The order merging logic currently is not located in API, so there is no way you can obtain data of the resulting order without actually merging a set of orders by calling the IEngine.mergeOrders method.
While only the resulting order amount is concerned, the order merging algorithm is rather straightforward:
[SELL,0.2] + [BUY, 0.5] = [BUY, 0.3];
[SELL,0.2] + [SELL, 0.4] + [BUY, 0.5] = [SELL, 0.1]
[SELL,0.2] + [SELL, 0.3] + [BUY, 0.5] = [] (the merged order doesn't get created since the resulting amount is 0)

However, the price calculation algorithm is more evolved.

Also see order merging preconditions:
https://www.dukascopy.com/wiki/index.php ... _Positions