This is easier to understand with a picture than text, but here goes. Your basic description is fine, as far as I can tell, but let me clarify a couple of things.
Unless your fan is sharing a circuit with other loads, a 12/2 cable is oversized and indicates a 20 amp circuit. Many home interior lighting circuits are 15 amps, so given the extra cost of 12ga wire and the PITA factor of workability, I would go with 14/2 wire, AS LONG AS THE CIRCUIT IS ONLY 15 AMPS.
In your J-box for the switches, your "supply" cable enters the box, and does this: 1. the white wire is nutted together with all of the other white wires in the box, just like you said. 2. the bare copper ground wire is nutted together with the other copper wires in the box, AND two more 8" or so wires each connected to the switches if they have ground screws. 3. the black wire is nutted together with two 8" or so black wires, each running to a switch.
Each switch will have a black wire running off to its respective load, (lights and fan).
If you want to add a timer, purchase a timer switch instead of a regular "light" switch.
If you want to add a "humidistat", you may add it anywhere between the switch and the fan, wherever you think is appropriate. You may treat the humidistat in one of two ways, depending on what you want to do:
1. Don't add a switch at all to the jbox, just allow the humidistat to do its job when needed.
2. Add a switch to the jbox, giving yourself the control option of keeping your bath fan from running when you don't want it to, and being able to leave your bath fan on, but knowing it will shut off on its own when the fan has accomplished its job. *my preference*
your choice here is also affected by cost, I haven't priced humidistats to know, but I bet they cost at least a few samollies. The cost benefit ratio is one only you can decide.
Please be extremely careful and TURN OFF YOUR CIRCUIT before wiring begins. Use caution in your attic and in using your drill.
TIP- When you decide to drill through your plates in the attic to fish your cable, do this first: Take a wire coat hanger and poke it through the ceiling at the corner where it meets the wall, at the point in the wall where you want to have the wire come down. Make sure you know which side of the jbox the stud is on so you don't take the trouble of fishing the wrong bay.
The coat hanger should poke up high enough to make it easy to locate through the insulation. When you are finished, a little caulk should hide the hole and you won't even know it was there.