As far as unloading, will depend on the length of the site, in a long one you won't have to unhook or unload. Dollies are usually light weight and can be moved easily by hand. You will want to get a dolly with either surge brakes (easiest) or electric brakes, it's the law in almost every state to have some type of braking system.
Backing up can be tricky but it can be done, carefully, since the pivot between the ball and wheels is very short. A good quality trailer lock like a Master Lock is a good theft deterrent, nothing is foolproof!!!
Sorry, but can't help with the kayak answer, guess it depends on if the bow extends beyond the nose of your car.
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