Good afternoon experts !

My large house in France has a 65 kW De Dietrich oil-fired boiler feeding 16 cast-iron rads. As you can imagine we use it as little as possible. The principal flow/return iron pipework, 60 mm diam (no, not 63 or 65) ascends from the cellar to the roof space some 15 m (50 ft) above, from where the rads are tee'd off in copper from the main distribution ironwork back down to the ground and first floors. It is sealed. All well and good apart from the running costs.

In addition there is an air to water heat pump dating from about 1983 - a pair of Saunier Duval MSDX 151 monobloc exterior pumps, 4.9 kW output each. It has never worked since before 2004, because the previous owner's plumber or electrician messed around with the control unit (satellite SP 22) wiring; also the internal pump was defective. So I replaced the pump, and was able to temporarily get each unit going again by bypassing some of the wiring. I even found a manual on the internet. They produce hot water for a few minutes until something cuts in to switch them off. So the refrigerant circuit seems to be intact - fortunate, because its R22 gas is now illegal to replace or replenish in France.

I could spend a lot more time understanding and re-installing the wiring - I hate the idea of throwing away kit that apparently still works - but the pumps are rather dated; for example, they don't function at an air temperature of below +7C. So I am thinking of replacing them with a 14 kW Mitsubishi monobloc pump (PUHZ-HW140VHA2) and control box (EHPX VM2C). Naturally I want to re-use the extensive 20mm copper piping from the pump/control location to the boiler room.

My main question is this: does the existing flow layout at the boiler as shown in the labelled photo below make sense (external pipe diameters in mm; Fe = iron, Cu = copper)?
pipework at boiler.jpg
As you can see the heat pump flow and return are BOTH on the main return, which seems counter-intuitive to me. But in fact this corresponds correctly to the Saunier Duval diagram below:
pipe scheme.jpg
The heat pump connections to the existing circuit shown in the diagram above are enclosed in a dotted rectangle, which is also labelled as where to put an optional buffer mixing tank (which I don't have). The French instructions also specify that the main pump must be on all the time.

The three-way valve shown above the boiler is a hand-adjustable mixing valve. The main pump is a standard Grundfos UPS 25-60, which might seem to be rather feeble given the size of the installation, but seems to work fine at its high setting.

Can I just use this existing pipework, but exchanging the mixing valve for a three-way valve to isolate the boiler when the heat pump is running alone? Should I change the heat pump flow to connect to the main boiler flow (which would be annoying, because I have never done iron plumbing before)?

Your advice and any other comments would be gratefully received.