let's guess whe performed characters who had their voice dubbed

[Xerus]

There is a 'backstage' scene in the Roger Moore episode where Kermit is having a conversation with a Viking Pig who has just come off-stage (form doing 'The Viking Number'). Judging by the photograph in the book OM&M, that character is being voiced by Jerry Nelson, but puppeteered by Louise Gold. - But goodness only knows why.

I also noticed in the making of that viking number, it showed a villager singing, "Come on and join your fellow man", in Jim's voice. But then it showed Frank operating the same Muppet.

 

[minor muppetz]

I also noticed in the making of that viking number, it showed a villager singing, "Come on and join your fellow man", in Jim's voice. But then it showed Frank operating the same Muppet.

The voices for songs were always recorded before the scene was shot, so the performers would have recorded voices and it wouldn't really matter if the same performer doing the voice was doing the puppetry, since the hole thing was being lip synced.