Lucky you - I'm sure you will enjoy your trip.
Your itinerary looks good and will certainly allow you to see some of the best of the 'real' Tobago.
In answer to your specific points:
1) No, not really. The market can be very vibrant and interesting, but personally I wouldn't waste valuable holiday time to fight the traffic and exhaust fumes of the town.
2) There is no compulsion to take a guide to visit Argyle Waterfall, although you will be asked to pay for one at the entrance. The big problem will be persuading the guides there that you don't need or want one.
3) You will not be able to just call a taxi from Parlatuvier, as there are none based in the area. The same applies to all the destinations in your itinerary. A self-drive car would be your best bet, but if you don't drive then that's not an option obviously. So, my advice would be to pre-arrange to hire the same driver and taxi for the duration of your stay. The owners of your accommodation should be able to recommend and arrange someone, or you could wait until you arrive and choose someone at the airport - but that is a lot more risky. Just because a taxi driver can take you to any hotel in the south-west of the island doesn't mean they know the coastal areas you are wanting to visit. So, speak and arrange a car through the owners of your guest house would be my advice.
myTobago.info - the definitive Visitor Guide to Tobago