Electrical question concerning GFCI
The power that's intended for your bathroom stays in your bathroom!

First, the answer to #1 is yes, you are alright on that front -- the GFCI only has to protect the bathroom receptacles, not anything else on that circuit, as per 210.8(A)(1):


  210.8 Ground-Fault Circuit-Interrupter Protection for
  Personnel. Ground-fault circuit-interrupter protection for
  personnel shall be provided as required in 210.8(A) through
  (C). The ground-fault circuit-interrupter shall be installed in
  a readily accessible location.
  
  Informational Note: See 215.9 for ground-fault circuit-
  interrupter protection for personnel on feeders.
  
  (A) Dwelling Units. All 125-volt, single-phase, 15- and
  20-ampere receptacles installed in the locations specified in
  210.8(A)(1) through (10) shall have ground-fault circuit-interrupter protection for personnel.
  
  (1) Bathrooms


However, the answer to #2 is no -- neither the CO detector nor the hall receptacle belong on the bathroom (technically, bathroom receptacle, but many bathrooms invoke the exception to 210.11(C)(3) that allows the rest of the bathroom's outlets to be on the same circuit as the bathroom's receptacles provided that the circuit only serves one bathroom) branch circuit.  It's a 210.11(C)(3) violation:


  (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).


So, I'd find another circuit to tap for both the new CO detector circuit and the existing hallway receptacle.