Why does the Humility/Opalescence interaction work?
You are correct on both points.


The entire ability will be applied in the appropriate layer, as mentioned in the first part of Opalescence's gatherer ruling :



  [...] the type-changing effect applies at layer 4, but the rest happens in the applicable layers. The rest of it will apply even if the permanent loses its ability before it's finished applying. [...]



Opalescence's type-setting ability works because it's applied in an earlier layer (layer 4) than Humility's abilities-removing (layer 6).


Edit : Also works because Opalescence's doesn't affect itself, as Benjamin Cosman mentioned.