For question No.1, you can see the document in the following link, which has a very good explanation about Traction motor suspension and the advantages/limitations conducted on IR locomotives.
click here
Firstly, Gear ratio is not related to type of traction motor being used ( DC or 3 phase Induction). It just indicates what the locomotive has been designed for. For identical traction motors, higher GR will result in higher speeds, mainly used in Passenger locos, while lower GR means more torque, which is usually for Freight locos. Also,...
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number of stages in the gears also does not indicate any performance impact. As long as the gear ratio is same, implementing it in 2 or 3 stages doesn't make any difference.
The G9 and P7 have traction motors partially suspended on the wheels, thus they are locked onto the wheel axle ( as in very less danger of misalignment between the wheel axle and the motor axle). So, a 2 stage gear box is enough. In a P5, the motor is completely suspended on the bogie and is not locked onto the wheel axle. So in case there is any misalignment between the wheel axle and the motor axle due to track conditions etc, there is a danger of the gears losing contact. So, a flexible intermediate gear is provided to take care of this. Also, the G9/P7 motor arrangement is very compact while the P5's arrangement is a bit larger. So another minor reason for a 3 stage gear system is the larger and variable gap between the wheel and the motor. Again, these numbers cannot be used as a benchmark to say which loco is better. The document in the above link has a more clear explanation.

The features that make 3 phase locos as 3 phase are the Traction motors alone and nothing else!! Even regenerative braking is possible with conventional locos. (Technically possible, but with a few limitations)
For the same output power, induction motors are usually much smaller than DC motors. Thus 3 phase locomotives are more attractive because it is possible to design higher power locomotives in the same body frame, without exceeding the Axle load limits. Also, due to the latest power electronics advancements, it is much easier to computerize the controls in a 3 phase drive. All the other features like HOG, Regenerative braking, computerized control, Dynamic braking etc are not exclusive...
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to 3 phase locos and can be designed for conventional locos also.

1. Motors alone! Gears are merely meant for transmission and control of torque generated by the motors.
Stage in transmission means the number of times the reduction needs to be done to get the desired output.
In single stage gearboxes only one reduction is there (2:1,4:1, whatever). The input rpm is reduced only once, in a single step.
In two stage gearbox, reduction is...
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done in 2 steps. Say the input rpm is 4000. In the first gear it is reduced to 1000 (1:4 reduction). In the next gear it is 3000 (3:1 reduction). This is two stage conversion.
Similarly there is multi stage conversion.
2. Both locos are doing well in their respective domains. P5 is a good loco and can handle 24-26 coaches too but has some limitations when the ability to start the load on a gradient (steep ones) is considered. The main purpose of P7 is to overcome these limitations (but at the expense of speed). In fair weather conditions, P7 can climb the ghats between NGP and ET without bankers with 25 coaches (max). Other passenger locos (including WAP5) will get bankers once the load exceeds 18-21 coaches. Also, the production of P7 gathered pace whereas P5's production rate is still yet to increase, mainly because of complicated motor assembly & 3-stage gear assembly of P5 and the similarities of P7/G9 with conventional locos.
3. It's called multiple-unit (MU) operation of locos. In this case the lead loco is the "master" loco and the trailing loco is the "slave" loco. The crew drive both locos simultaneously from the cab of the master loco. MU cables which connect master & slave locos are meant for transmitting communication signals from the master loco to the slave. Traction is delivered by both locos simultaneously.