No, like all other Pokemon games you can only have 1 save file.
At one time, no.
Mt.Moon
There is no best Pokemon on Pokemon HeartGold.
there is no Pokemon Mansion in Heartgold.
no
no
You have two beat the Pokemon league.
You catch Mew-Two
there is only two regins.
They are not in Pokemon HeartGold
you can get it by getting a pichichu in pearl trade it to heartgold
put two Pokemon in the daycare 1 female and 1 male