I have to make the wilton castle out of vanilla chocolate to stand on top of the cake. Is it ok to let it out on my counter without melting? My house has central air about 72 degrees will it be ok?

Chocolate doesn't melt at 72º so it's fine to set out on your counter.
But beware of chocolate eating gnomes!!! LOL

How far in advance can I make this Candy mold?

Pretty much as far as you want to, month, 2/3/4/months. It's just chocolate
and if it's kept at a good temp, it will be fine. But I would put it in a covered
box or under a cake dome to protect it from dust, etc. Hope I've been of
some help.