the "z" was retained here (though lost in "yoke" itself).

Actual, it was added here. It never was in yoke. It's the zero-grade form of yeug-.

http://www.bartleby.com/61/roots/IE600.html

That would imply "yoke" and "yolk" are connected.

You might infer that but you'd be wrong. Yoke is related to yellow

http://www.bartleby.com/61/roots/IE158.html