1. Is it necessary to make a reservation to visit port quintas?
For most, yes. You can get a brochure at the tourism office with the quinta's participating in the Port Wine Route. In there you can find if you need to make a reservation. Not all quinta's participate in the route, however. So, if you want to visit a particular quinta, it is best to call in advance. Some quinta's do not allow visitors at all (e.g. Noval, all quinta's of the Symingtons), so unless you have connections there, they are impossible to visit.
2. Which of the port quinta's on the Douro are absolute "musts" to see?
If you can get an appointment: Quinta do Crasto.
If you have connections: Quinta do Vesuvio, Quinta dos Malvedos, Quinta de Vargellas, Quinta do Noval (those are all on my 'wish list'

)
If you want it the easy way: Quinta de la Rosa, Quinta do Portal, Quinta do Panascal, Quinta do Passadouro. All offer a tour and provide a good tasting room, although Panascal is very touristic and doensn't offer anything interesting to drink.
3. Besides enjoying port, what are any other "musts" within a one hour drive from Pinhao?
Pinhao isn't exactly the most exciting place on earth. You can take a boat tour on the river, or take the train along the Douro, either in the direction of the Douro Superior or in the direction of Regua / Porto. Both beautiful.
In Regua (also not a very special town) you can visit the Solar do Vinho do Porto, where they have some fine ports on the list, and it includes a nice excibition on the history of port wine.
Near Vila Nova do Foz Coa (in the Douro Superior, about 1,5 hours drive) there is the archeological park of Coa, where rock art of about 10,000 years old can be seen. You can visit it by guided tour (reservations needed). You can also combine your tour with a visit to Quinta da Ervamoira (of Ramos Pinto), that has a museum with it. You can ask for this while you make your reservation for the tour in the park.
4. If possilbe I'd love to "stomp" some grapes. Does anyone know the best way to make arrangements to do so and at what quintas?
I don't know much about that. If you call quinta's for appointments, it is probably best to just ask about it.
But I don't know if there is still a lot of harvesting going on early october. Because of the heat and drought, the harvest started already in the first week of september at most quinta's. But this varies of course between different sub regions and quinta's, so it is best to just inform what the planning is if you try to make an appointment.
Hope this helps a bit. Have fun!