Answer:
The snake spoke to Harry.
Because they think he was the one who let the snake-free, and he was also the one who traped Dursley in the Glass dome.
The Dursleys treat Harry very cruelly in regards to his magic because they blame it for the death of his parents. Both, including Petunia Dursleys' younger sister Lily, were killed by Voldemort as young adults when Harry was just a baby.
His hair was cut and grew back overnight; his sweater shrank all on its own, and he was able to leap onto a chimney.
Just random wizards and witches that have heard of Harry Potter, but who don't know him personally - and of course, Harry Potter doesn't know them. The idea is that Harry Potter is famous, due to his defeat of Lord Voldemort when he was one year old - of which he remembers hardly anything, and (at first) has no idea how it might have happened.
