<<"The Black Forest and Rhine Falls tour starts at 12:15 Hrs & ends at 19:15 hrs at Sihlquai Bus Terminal, central Zurich.
Can we reach the airport from Sihlquai Bus Terminal, central Zurich to Airport on time to catch the flight ?
If the airport is on the way back route of the Black Forest and Rhine Falls trip?
If we can carry our luggage at the bus of the trip Black Forest and Rhine Falls ?">>
You can make it to the airport by 2000, for your 2200 flight, assuming you can transfer luggage or pick up luggage from the Zürich main station storage. (Careful, not sure what time the left luggage office closes; but, if you have a locker(s) - no problem).
It is possible , but needs careful attention to the timetable:
<<"If the airport is on the way back route of the Black Forest and Rhine Falls trip?
If we can carry our luggage at the bus of the trip Black Forest and Rhine Falls ? ">>
I do not know how to tell you if the tour returns by a route passing the airport, since I do not know the details of the tour, or whether you can carry luggage, or , for that matter, how much luggage you have. Perhaps you could ask the tour operator?
Each of us has our own perspectives on what is "worth it."
So, my opinions may not match your interests.
That said, my opinion is that spare time near Zürich is better spent at Luzern.
Since you have not defined the alternatives, I can say -
"Skip the tour."
However, if you give us more details on what you are doing, perhaps we can offer a better answer.
Basically, the Rhine Falls are better than nothing, but not as good as many other things to see.
Where will you be on your tour before that, so we can suggest comparisons or alternatives?
Any sight or place in Switzerland has to be compared with other excellent options in order to make a choice.