Pipe 22mm and quite long pipes; but boiler now range rated at 18 kW so that should be ok I guess. One more question to the person that put this oversized boiler there...
The limiting factor when considering pipe size is the velocity of the water (metres/sec). If it is too low there is a danger of sludge etc accumulating in horizontal pipes; too high and the water can be heard flowing through the pipes. A velocity of about 1metre/sec is normally considered OK. A 22mm pipe can carry 14.7kW @ 11C boiler differential and 26.8kW @20C boiler differential without exceeding 1 metre/sec. The difference is due the different flow rates (litres/sec) required for 11C and 20C differentials.
The pump and ABV are the same on all versions of the 637, so your graph is the correct one.
Yes, the flow rates given in the tech data are those required to provide a 20C differential at max power. This does not mean you have to maintain this rate all the time, which is a mistake frequently made by many. It would probably be better if manufacturers stated the maximum permitted differential across the heat exchanger.
If you look carefully at the graph you will see that the Y-axis is labelled "lift". This is the head available for overcoming the resistance of the heating system, i.e the index circuit. If the bypass is set too low there will be insufficient head to circulate water through the index rad; if too high there will be excess head, which will cause the bypass to open. The horizontal lines are similar to the constant pressure lines on Grundfos Alpha pumps.
I have shown on the chart the flow rates for both 18kW and 6.5kW at 11°C and 20°C differentials. If the flow rate remains constant the differential will reduce in direct ratio with the change in output; i.e 20°C differential at 18kW will become a 7.22°C differential at 6.5kW.
"Restricting the flow" is the wrong way of looking at the problem; the task is to provide the
correct flow through a radiator. So if you have a 22mm pipe carrying water at 775 litre/hr (18kW @ 20°C differential) and this is fed to 18 1kW rads, via 15mm connections, then each rad will require 775/18 = 43 litres/hr (0.72 litre/min, 0.012 litres/sec). Given that a 15mm pipe can carry 12kW (20°C, 1.0 m/sec velocity), a lockshield valve will have to be closed down a lot to restrict the flow rate to what is required through the rad. Water is not pushed through a rad by a pump, it is sucked through because the pressure at the return connection is lower than that at the flow.
The simple answer to your question is to balance the system so you are working in the flat part of the curve and the differential does not exceed 20°C. This must be done with all TRV heads removed, pump set to operate in stage 1 and bypass adjusted so the furthest rad is heated.
does not appear to be the case in your world. 
