GPS signals need line of sight to the satellite, so positioning the antenna on top of the aircraft maximizes the satellites that can be seen and minimizes multipathing, and also gives some protection to ground based interference/jamming.
Not listed in the CAE but there are some advantages in not having critical cables running half the length of the aircraft, so the location may be the one that got a cheap, simple and easy to install and test location for the antenna, and per below it may not make a difference for this particular nav system.
Keeping the antenna near the C of G has two two effects, the first is that it means the position output is near the actual aircraft center, avoiding issues with systems having it off the side of the taxiway or showing altitude while still on the runway. There are also several effects the reduce the precision if there is rapid motion of the antenna during fixing, so a central location will get more accurate position. All of this can be 'solved' with software, but does make accurate GPS dependent on accurate pitch/roll information (and good software). Presumably in this case software was traded against the other effects of central location to produce the current placement.
Two antennas would be for redundancy. While aircraft can operate without GPS, it generally increases crew workload which produces risk of some other problem causing a crash.
All GPS signals are line of sight and most receivers use all
satellites that are visible and a minimum of 9 - 12 degrees above
the horizon. So being on the top of the aircraft near the centerline
is essential to providing the best reception.
It's somewhat arbitrary as all antenna locations are a tradeoff. All
the various antennae have to separated to avoid potential
interference and parasitic effects. Cable lengths are an issue for
two reasons. Signal attenuation and weight. Most of the antenna
cables end up routed to the E/E bay aft of the nose gear well where
the majority of the avionics are located (HF and radio altimeters
are located in an aft bay.) I don't know why this diagram shows 4
GPS antennae, 2 MMR and 2 'GPS'. I only know of the GPS receivers in
the MMR. Every aircraft will have some variation is locations. The
biggest known interferer for GPS is the VHF com, so keeping them
separated is essential. Forward locations will avoid any shadowing
by the vertical stabilizer/rudder.
The aircraft C.G. is the reference point for the navigation system.
It's the point about which all aircraft rotation is measured. As the
nature of the GPS solution is time difference of arrival of the
signals from the satellites, the resolved position is that of the
GPS antenna. The antenna location offset from the aircraft C.G. is
loaded into each GPS(MMR) to allow the GPS to compensate for the
offset. Additionally, the GPS data will be merged with the IRS data
(which has its own compensation for installation location) in the
FMS to obtain a hybrid GPS/IRS position. So ultimately any offset
from the C.G. is accounted for. The only benefit of being close to
the is to reduce 'noise' due to aircraft rotational motion
(pitch/roll/yaw). Since it will always be present with even small
offsets, there is an internal Kalman filter to reduce it to a
Navigation is an essential function whose loss is considered Major
or Hazardous depending on phase of flight. In either case it
requires redundancy so there are two of everything - GPS(MMR), FMS,
etc. There may be 2 or 3 IRS depending on the system architecture
and type of flight controls. Antennae can be shared between systems
if they are simple antenna and meet a minimum reliability. Because
GPS antennae are complex, containing an amplifier and downconverter,
each GPS receiver has a dedicated antenna.
Specific to question 2, that has not been addressed in other answers… The GPS MMR antennae receive GPS augmentation signals. In the case of WAAS, which is provided by stationary satellites, (SBAS), you want those on top. But MMR augmentation also includes ground-based signals, (see GBAS), which are transmitted from the airport. GBAS airports are rare today but are likely to become more common over time. So the logical location that is line-of-sight to both satellites and the runway while on final approach is above the windshields.