Is it Niranjan who causes dissolution?

Yes, This job is performed by Niranjan. Param Purush has nothing to do with all this. All sorts of dissolutions—big or small—are caused by God-Niranjan. Some think, God has given them Salvation. Actually it is not so. He who has been killing all, can in no case be expected to give salvation to anyone. His sole job is to kill the Jivas, to eat them and then to give birth to a new creation.