As I read it -

does refer to the address interpolation way. Otherwise there would be extra steps required to locate the address interpolation way in software:
collect all ways spatially close to the street.
Inspect for type address interpolation and end nodes part of the relation.

Also if the address interpolation way is ever deleted in the future, it will also be deleted from the relation.

I think I understand about how the selected nodes would work. So I think the following changes would apply:

  1. Always create using a relation (no choice for the user).
  2. Scan all selected nodes and all nodes on the address interpolation way for addr:housenumber, and automatically add to the relation if not already present. This can be done at the same time the address interpolation is created, or later.

For the case where just nodes are added later to the relation, it is functionally the same as adding them to the relation with an automatic role=house.