D & C 114 was not a prophecy, it was a mission call. Joseph Smith, under the inspiration of the Lord, issued a call for David Patten to go on a mission the following spring. This call by revelation is not a prophecy that David would serve a mission, but an admonition to set all his affairs in order so that he may perform a mission. Although Patten was killed, his affairs were in order when he died so that his family could endure his absence. This alone indicates the Lord's foreknowledge of Patten's death. And who knows but that Patten served that mission call on the other side of the veil?
In any event, Patten's death would not change the instructional nature of that call. Joseph Smith declared that: To the "great Jehovah . . . the past, present, and future were and are, with Him, one eternal 'now'" (History of the Church, Vol.4, p. 597). The Savior does know all that will happen to us individually, but he still gives agency to us and to others who impact on our lives, which usage often precludes what would have happened if the Lord's will were done on earth as it is in heaven.
There are several Biblical parallels to David Patten's mission call, such as the calling of Judas as an Apostle. As one of the Twelve Apostles, Judas was promised by the Lord that he would sit on twelve thrones with the others and judge the twelve tribes of Israel (Matt. 19:28). Judas, of his own choice (unlike David Patten) never fulfilled this promise of the Lord. This doesn't make the Lord a false prophet in the case of Judas. Nor were the Lord and His prophet, Joseph Smith, mistaken in the case of David Patten.
The Lord knocks at the door and gives the promise or opportunity. Whether we open the door and respond in a way to reap the potential blessing is up to us, and in many cases, up to the righteousness of others. In David Pallen's case, extenuating circumstances prevented him from serving an earthly mission: a mob killed him. To understand the case of David Patten, one might study D & C 124:49, which states if "their enemies come upon them and hinder them from performing that work, behold, it behooveth me to require that work no more at the hands of those sons of men, but to accept of their offerings."