Appointing a two year place holder had more than just political risks-- Robert Byrd assumed room temperature with more than six months left on his term. Under those circumstances, West Virginia requires an actual election to fill the vacancy.
Manchin would have not been on very questionable legal ground had he tried to count the six months from the time he actually declared the vacancy, a couple of weeks after Robert Byrd was dead.
Some Democrat operatives were actually urging that he take that route to avoid the risk of a special election. But in the end, Manchin did the right thing by calling the special election. I give him credit for that.
Some people felt that there was enough wiggle-room in the law to allow Manchin to avoid calling a special election in order to save the seat for Democrats. There are several threads here on the subject (Google site:freerepublic.com robert byrd special election).
It was interesting to watch West Virginia come to the decision to hold the special election, and then watch how the "sure thing" evaporated.
-PJ