in the Japanese version there are events. But in any other versions its impossible without Action Replay or other cheating device. Or going to the Pokemon convention lol.
To get a Mew in Pokemon Heartgold, you could do it through a Nintendo Wi-Fi event.
Yes you can if you have Mew in your Pokemon SoulSilver/HeartGold You can put Mew in the Pokewalker.
An event
yes
yes
Mew
mew
Mew can't be catched in Pokemon heartgold, only mewtwo.
you cant for heartgold or soulsilver suckers
Nintendo event
you can't
You cannot get one.