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.
He could return to his appearance actually. In Darkman 3 he did perfect the formula and showed it by dipping his finger tip into the flesh and it reformed onto his finger tip. He used the rest to heal the kid's face damage.
But if that perfected skin is still on his finger then he can analyze and replicate it.
However even if he restores his old face with the new skin, he is legally considered dead and also he has some mental issues. He really can't return to his old life.