Hi Juliette
Welcome aboard!
My, I don't know what/where you've been getting your information from, but you seem to have a rather negative view of Tobago. Dismiss those concerns and read the information on this site for the TRUE position.
Anyway, in answer to your specific questions:
1) Yes, if you leave your car/jeep unlocked and valuables clearly on display, the chances are they will be stolen (just as likely to be stolen by a tourist as a local). Otherwise, car theft is virtual unknown on Tobago.
2) Sweltering temperatures? Well, an average temperature of 31°C (88°F) is hardly 'sweltering' in world terms, particularly not when cooled by those wonderful Atlantic breezes that Tobago benefits from.
3) See our article in the Tobago/Getting About menu for details of car rental. We always book Sheppy's via Yes Tourism.
4) If you book tours with any of the tour operators mentioned on this site you will NOT be ripped off. If you hire people on the beach, you have only yourself to blame.
5) Most hotels allow non-residents to use the restaurants and bars.
6) Topless sunbathing and nudity is illegal and highly frowned upon everywhere on Tobago and you will cause great offense to the older population if you attempt to do so.
7) Local produce is very reasonably priced. Imported European foods are expensive. By sticking to local produce and fish, you can eat extremely economically.

Loads of spending money? How long is a piece of string? If you are going on excursions all the time, it can be expensive. However, you can have really wonderful holidays on Tobago for very little cost.
9) Hummingbird Heights has a very good reputation. We have stayed there and you can read our review, available through the Reviews/Accommodation menu. Hummingbird Heights also has its own section in the Accommodation section of our forum where you can read comments from other visitors.
Don't worry - you're in for a treat.