Electrical – add a garage outlet from existing interior circuit

If you don't plan on using any 20 ampere cord-and-plug devices, there is no reason to install 20 ampere rated receptacles. Circuits rated for 20 amperes are useful in the home, because they can support more devices. 20 ampere receptacles are often not advantageous, except possibly in a workshop or garage.

You are correct that the first two options are allowed by Code (citing from the 2014 NEC here) -- the relevant passage is 210.11(C)(3) along with its Exception:

(3) Bathroom Branch Circuits. In addition to the number of branch circuits required by other parts of this section, at least one 120-volt, 20-ampere branch circuit shall be provided to supply a bathroom receptacle outlet(s). Such circuits shall have no other outlets.

Exception: Where the 20-ampere circuit supplies a single bathroom, outlets for other equipment within the same bathroom shall be permitted to be supplied in accordance with 210.23(A)(1) and (A)(2).

However, this passage only applies to receptacles unless you take advantage of the terms of the exception; putting the bathroom light on a shared lighting circuit is Code-legal as long as the receptacles are on a dedicated circuit.

For the heaters, you can either put them on a per-bathroom circuit as long as they satisfy 210.23(A)(2) by not using up more than 50% of the circuit's ampacity:

(2) Utilization Equipment Fastened in Place. The total rating of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch-circuit ampere rating where lighting units, cord-and-plug-connected utilization equipment not fastened in place, or both, are also supplied.

or you can have them on a shared lighting circuit, if you decide to have a separate circuit for bathroom receptacles. Unfortunately, putting them on the same circuit as the bathroom receptacles when that circuit is shared between bathrooms is prohibited by the Exception to 210.23(A):

Exception: The small-appliance branch circuits, laundry branch circuits, and bathroom branch circuits required in a dwelling unit(s) by 210.11(C)(1), (C) (2), and (C)(3) shall supply only the receptacle outlets specified in that section.