Although we do not know what happened to Captain Luc Valonforth. It is, indeed, true he remained loyal to the Alliance during the course of the whole journey. He answered to King Tereneas, rather then Prince Arthas. But then again, wouldn't anyone after they realized their Prince is acting somewhat out of the ordinary? BUT there are some things to be noted.
However, interesting fact is that Luc Valonforth appears in the flashback quest "The Truth Shall Set Us Free", where he re-enacts the scene involving the Alliance Emissary who ordered the troops to be recalled to Lordaeron. Unlike the rest of the Forgotten, he does not turn into a ghost at the end of the quest - he just vanishes like Muradin and Arthas, suggesting he did not die there. This was proven by Muradin who simply got shot by the shard of ice holding Frostmourne. He woke up, lost his memory, wandered the wastes, and etc. Arthas became the Lich King afterwards. So both of the non-ghosts vanished and were proven to be walking among Azeroth as the quest reveals. But whatever happened to Luc Valonforth after those events is unknown. However, what IS known is that Luc Valonforth still walks the earth of Azeroth in the unknown... Something that Blizzard pointed out clearly and wanted us to know. He is alive, but as an undead or living unit?
1. Living? Still unclear. But if he was undead, I think we would of seen him by now. Therefor, I believe that Valonforth was in fact the only and sole survivor of the expedition. Being the only capable soldier realizing Arthas' descent to madness. He wanted to leave. He wanted to obey King Terenas. He surely did hear about the incident in Stratholme. Even more proving why he wanted to leave. But that still leaves us confused about what happened to him after the events of "Betrayal".
2. If he truly did get killed by his own people. Then Luc Valonforth was risen into undeath, but surely wasn't seen afterwards. Marwyn and Falric were seen to take care of Lordaeron. But what part did Luc Valonforth have in all this? Where did he go? Where is he now? Another speculation left unknown, but maybe he is doing grievous and more sinister secret tasks? We can only hope.
About the Model:
I always imagined him more royal looking then anyone of the Captains and somewhat of an "outcast" with the group that set sail to Northrend. Since he is one of the Captains that doesn't obey Arthas, but rather Lordaeron. The model described him perfectly and fits with the above. Another spectacular model.