Would they have gotten away with it if they were more shrewd?
I believe that in the short term, absolutely, but they would surely be colllared in time.
If Brandon wasn't so cocky and Phillip played it cooler, and both played up more concern for David's absence in the rest of the company's presence I believe they would have been able to prevent arousing suspicion long enough to at least carry out their plan of disposing of David's body in a lake.
Brandon was at least shrewd enough to realize that an extended absence from their home would generate too much suspicion, but I don't think that would be enough to prevent them from being suspects. At that point if there is no body, there is no murder but the authorities would most certainly begin grilling everybody, Brandon and Phillip especially. Obviously Phillip would fold under questioning first, but Brandon's stuttering would cause pique their interest to press harder yet and he would fold as well.