1. The day/night cycle is handled by daynight, I believe. In dayNight.src you'll find a function called transition that sets the light levels.
2. The logic that checks to see if a particular place is a valid spot for a player vendor is done in /scripts/items/vendorDeed.src. You could add your own custom tile and add some logic to vendorDeed.src so that it would be a valid location.
3. I don't believe it's possible using the current distro but it's not that hard to implement a change password script. You would just use something like this:
Code:
var account = who.acct;
account.setpassword("newpass")