Electronic – Current limiting resistor on peltier element

If your heater is only resistors and you're going to PWM them anyway, then connect them directly to the 12 V supply. The linear regulator will only waste a lot of power and make heat that you have to deal with. Arrange the heater resistors so that the heater gets a little warmer than you need with 12 V applied. You then use PWM to throttle that back.

For the Peltier, you don't want to use pure PWM. That is not only much less efficient, but can also damage the Peltier. In that case, you want to use a switching power supply. That can be as simple as the PWM you show, but with L-C filtering to keep the current thru the Peltier reasonably constant for any fixed PWM duty cycle. The general rule of thumb is to keep Peltier current ripple to 10% or less of the average. The temperature feedback closes the loop, so you don't have to explicitly regulate volage or current thru the Peltier. The control loop adjusts the PWM duty cycle according to the temperature feedback.

There is more information about driving Peltiers from a digital PWM signal here.

The voltage rating is for the resistor series typically and specifies the maximum peak voltage you can apply without danger of damaging the resistor due to corona, breakdown, arcing, etc.

The power rating is completely independent of the voltage rating. It specifies the maximum steady state power the package is able to dissipate under given conditions.

You have to conform to both specs. If placing the maximum voltage across the resistor results in more power than the spec allows you have to reduce the voltage until you meet the spec. Likewise you can't increase the voltage above the rating just because you're not hitting the maximum power limit.