Voltage Control of PQ Bus
There's no such thing as leading or lagging reactive power. You can increase or decrease the amount of reactive power you introduce in the system, by inserting capacitors, coils, transformers, cables, filters or change the voltage/current angle of generators, inverters and so on.

The amount of reactive power is what determines if the current is leading or lagging the voltage.

The sign convention of reactive power is not really a thing, it's a positive sign if you inject it, and a negative sign if you consume it.

Where you might encounter problems with the Newton Rhapson's load flow is with the equation for real, reactive and apparent power equations that you calculate using the current and voltage:

 

The apparent power is the voltage times the complex conjugate of the current, meaning you change the sign of the imaginary part of the complex number (or change the sign of the angle). If you inject current with a leading angle, you'll get negative imaginary part, and vice versa.

Since the reactive power is the imaginary part S, the sign is opposite of what you get if you simply multiply voltage and current, which is the intuitive thing to do.