One caveat is that "self world" for a Morph will answer nil if the morph is not currently *in* a world.

The "self world" expression works for morphs, and certainly it is easier to read.
It may be somewhat slower, although that would not be a concern in most usages.

My main concern is correctness, because failures in this area can hang up the
UI entirely, and errors are difficult to debug.

When transitioning from one project to another the World variable is set to
the new project's world in #finalEnterActions:.  Thus the World global is a
shortcut reference to the world of the current project, and that is what leads
me to suggest the expression "Project current world".