You confused yourself here :)
I use xxxNext and xxxPrevious to qualify if there are next or previous "pages" of related data in one specific node (aka data in the model), while xxxNextNode and xxxPreviousNode are related to displacements done in the node stack.

Populate is used to add a node to the stack, while loadNext and loadPrevious are used to load related data for a given node. 

Calling populate is mandatory, because we have to be sure that the developer set the correct node identifier and set of filters before pushing a node associated to them.