[Grablevskij]

Let me try one more time.
About whichever:

Whichever one of you
Only one of them is meant.

Whichever of you
One, two, three of them or maybe the whole group is meant.

About the plumber:

A plumber is a person whose job it is to repair pipes.
It is to repair pipes that is a plumber's job.

A plumber is a person whose job is to repair pipes.
To repair pipes is a plumber's job.

Is is just an emphatic construction with a subject "it"?

Michael