It wasn't a suprise especially when Ordo Ad Chao was a let down. What more Blasphemer could do? IMO he didn't have "room" to evaluate in Mayhem. He did some experiments in GDoW and it sounded fucking bad. Chimera was a masterpiece in which he took a big part and after that I didn't expected something as much as awesome as Chimera.
As for the vocals I think that Maniac did a nice job in Mayhem, especially in WLA. In Chimera his vocals worked nicely with the misanthropic atmosphere.
Ofcourse it is only my opinion but if anyone thinks that Vargs vocals would do better than Attillas in DMDS they should have their fucking heads examined. Attilla perfectly fits to DMDS but he didn't do so awesome work in OAC.
Getting back to the guitarist issue I think that it will be a tough case for Mayhem to find a good substitution after Blasphemer. Hope they will find as solid guitarist as Blasphemer was.