To find what causes goblin girls to disappear, just advance in the area with bugs and slugs. I don't really remember the exact path, but hints are :
1) follow light on the ground (at some points light goes into 2 directions : one of them usually ends up a dead end quite fast)
2) eventually there will be no more light to follow, but you should continue. If you encounter flying platforms, you are on the right path.
3) go further. When you reach an area with thudnerstorm sounds, this is the area with a boss. Fight is not difficult if you are 100% healed. Just for info, normally Vicktor must avoid dying (Grubbe will abandon him), but with Boss, it is ok for Vicktor to die : Grubbe can finish it by himself.
As for becoming a goblin. You need to first solve the quest above
Then you will need to talk to Grubbe and get drunk with him. There is lots of grind involved (watching exact same unskippable scene around 20 times I think + having to wait 1 minute in real life between talking to Grubbe). Honestly, scene isn't worth grinding so much. If there was at least some variety of 3-4 scenes and not the same one...
The last scene is also nothing special unfortunately.