Hi Jayson, it seems to me that your surname name reflects the direct male line, so only a very small % of the DNA, which shows the full range of your male and female ancestors. For the Isle of Man, I would expect a mix of Viking and Celtic, which your DNA profile seems to reflect.
I would expect a crest to be related to a specific male ancestor or group of ancestors. I'm not sure I would expect to find "one" crest for the surname, as there could well have been many crests for different Costain families across places and generations. On the other hand, not all families had crests. To find one that may have been used by your ancestor(s), I would look at your ancestors on the Isle of Man.