Nepia

Most probably i have never tested a Mppt efficiency if not strictly adhering to the suggested Vmp range in this case between 60~105v. For me it is about practicality and cost for the installation.
Should you use 2S configuration to accommodate 12 solar panels in the array that means you need to configure you array as 2S6P hence the current running to your Mppt input would be ~78amps. You will need to use 10mm² solar cable with fuses and if long distance from Mppt you will need to use 16mm² solar cable to compensate for the voltdrop at that current. You will also need to fit 12 inline fuses to each panel as you are exceeding the series current flow of your panels by a great margin. 3S4P is by no margin perfect but a lower current would be achieved. Unfortunately in low voltage Mppt application's that is what you have to live with. Multiple Hv Mppt inverter is therefore a much better choice but again more pricey.
My 2c worth.

I fully agree on as you indicated. 
It does however not look as bad as 12 panels to the MPPT as I thought on would only wire 2S3P to a combiner box for the east side and the same for the west side. 
My guess would be that 6mm² could be used from each side. As I'm not an installer I could be totally off track and therefore always rely on you guys to help us out. 
If memory serves me right @Coulomb has mentioned the efficiency when going over the sweet spot for MPPTs on a number of topics. 

Today I did some testing .
To date I've only been running 1 string with two panels in series... while I'm still exploring where and how I'm going  to mount my other 10 panels.... This has been running 96volts around 12amps.
So First i tested the 1 string with three panels in series ....at 115 volts (earlier morning ) the fans kept switching on and off.( Zero load)
Then I went to 2 strings with two panels in series on each string.....the volts hovered around the mid 90s ...and around 24amps dropping up and down regularly ...and the fans stayed off .
At this time my wife had started cooking so not the best time I guessed to keep testing.
From what I've tried so far the inverter prefers to be happy at the 105volts as per the label on its cabinet.
Will carry on testing tomorrow if I can ( with pics)

1. Yes North panels to MPPT. Then the NW 4series go in parallel with the 4 series NE panels. 
2. On a 5kW I lean towards 455W but 545W can also work. Is is possible to fit 5 on the NE and NW if needed. 
3. Perhaps around 15% but who would know as Eskom tie their increase to reducing numbers using the grid to cover the cost of generating power. Not a great business plan going forward. 
4. Not just the speed(Hz) but need a stable speed when loaded. 
5. Starting perhaps at R180 000. Without a generator. A wild guess so let's get installers to guide you and get quotes. 
As you can adjust your habits and use if price is above your budget then start with panels proving the savings and move use to day time. You could start with less battery power as this is a high cost item and can be added if you start with 1x5 kWh. Time will guide you if you can go off grid. 
@TaliaB

I am not so sure about that. Identical panels yes! "The following rules must be observed in order to ensure that an east-west orientated PV system with a single inverter operates optimally: 
Shading must be avoided 
The number of solar modules must be identical in all strings. 
Within a single string, the inclination angle and orientation of the solar modules must be identical" extract" from attached paper.
east-west-solar-paper (3).pdf