The easiest work around for this is to make extension 100 NOT the attendant station. That way, unanswered calls to it will go to the mailbox just like any other station if you have it set that way in flash 50/51, page C.
You can get around having all of the phones ring for preferred line answer (PLA) by programming them to ring but then disabling the ring tone by dialing 695+36+ speed to save. Since most small offices use handsfree intercom signaling, this absence of ringing probably won't be an issue.
The day and night overflows you mentioned are all outlined in flash 40. You set the preset forward destination (440) for the CO lines there on page B, along with the desired mailbox number (100). Page C allows you to adjust the timers for preset forward for both the day and night modes.
Transferring a call to another person's mailbox is very easy. With the caller on the line, just press the voice mail button (this puts them on hold), dial the mailbox number you want (or press the DSS button for that station) and hang up. The caller will immediately be transferred to that person's mailbox.