Vosloo took over in Darkman 2 and 3 and both were direct to video.
Despite being direct to video they weren't awful to watch. As to switching Liam to Vosloo for flashbacks, well that way they only pay Vosloo.
Also if memory serves we were going to get Die Darkman Die first but the actor that played Durant became available so they gave us Return of Durant first instead.
Yes I believe for Evil Dead II they didn't have rights to the footage for Evil Dead so they reshot it. It worked better anyway because they had a new Linda. For Army of Darkness they actually used footage from ED2 but still reshot parts of it for brevity I suppose.
For Darkman II, I think they did use footage from the first movie but anytime you would have seen Liam Neeson's face they reshot it. I just thought it was unnecessary because his whole gimmick is switching faces anyway. But IMU7 is probably right about royalties.
Also, if they wanted to do a story where Darkman got to return to his original appearance I guess it would have to be Vosloo.