HBP: Chapter 8 - Snape Victorious

Convient Tonks came looking for Harry on the train and it was convient that Draco had pulled the shades so Tonks knew where to find Harry. Was it just luck that Harry didn't get left on the train?

Tonks is a strange character. I want to trust her. She seems like a nice enough person. But at the same time I am very suspicious of her. Maybe its the bubble-gum pink hair? In these books the color pink usually means something is being concealed. The Fat Lady in the pink dress conceals the entrance to Gryffindor tower and Hagrid's pink umbrella conceals his broken magic wand. What could Tonks be hiding?

Snape says Tonk's Patronus has changed (160). What was it before? We are never told. What made it change? Why did Snape say it looked weak?

Dumbledore's hand is still blackened and dead looking. Why hasn't it healed? Why hasn't he told anyone what happened to it?

Why did Dumbledore give Snape the position of Defense Against the Dark Arts this year? He knows the positioned is cursed and Snape will be gone by next year. Is this part of a larger plan? Why isn't Snape more excited about getting the position it's been claimed he's wanted for so many years? Was it really his "heart's desire" to get that teaching job or is that just what he wants everyone to think?