Some companies use them, some don't. Depends on your model's age. Most Ferris walk behinds did not.
Depends on your abilities and how sophisticated you want to get. Switch to switch to clutch will work.
KISS. While you're wiring the pto switch, probe it and find which terminals are n/c. You will use those when you get the interlock working. Sounds like the wiring on you're unit has been "Landscaperize"