Can SourceOrderId on a position always be used to find the id of the originating order?

Generally the answer is yes. However in a few situations this relation may break. One such example is if the position is subject to a corporate action. For example if an instrument changes from one name with one ISIN to another name with another ISIN, the Saxo Bank system will (over night) sell the position in the original instrument and buy an equivalent position in the new instrument. In this case the SourceOrderId will not be carried over to the new position.