There. FINALLY got Mamu and Nessa. After attempting all kinds of convoluted arbitrary nonsense, unassigning everyone in the village from their jobs to play with the bouncy red ball for a while, putting myself through hell with various weird long challenge runs in Infinite Mode, savescumming in order to hatch or discard Parra eggs at every single possible random event... I now feel absolutely ridiculous, on account of how simple the actual solutions are.
For Mamu, I looked up the solution, and I'm glad I did because it is something I never would have figured out on my own. Despite it being probably the easiest secret to do once you know how. Not going to simply repost it here, but I will give one more big hint on top of the ones that have already been said.
Hint: Empty Building
For Nessa... as far as I can tell, the complete solution isn't yet written down anywhere (and I'm not about to break that tradition), but once I pieced together the hints... I'm actually surprised I didn't get it sooner.
Important Info So You Don't Go Crazy While Trying Random Things: Infinite Mode is NOT required, and a specific zone is also not required.Hint: Rest