How big are the boilers? They can't be that big as they appear to be using domestic pumps. All that ironwork is NOT cheap. A simple off-the-shelf SS buffer cylinder with a stat would be a better solution, which you agree on, totally isolating the boilers from the system. It would also be much cheaper as well.
The Germans use buffers with a domed bottom, the reverse of the top, to prevent sludge build up. A good filter will stop the build up.
LLH do blend but a very crude way of doing it and the Delta T of the boiler not guaranteed. So, Heat X cracks may re-occur.
The boiler are 60Kw each, the two pumps in the pic are shunt pumps and no difference to the inbuilt pumps in the Kestons, they only need to circulate the water through the low loss header. In the OP case it could be considerable smaller, and many manufacturers supply an off the shelf LL headers, I know Valliant, Viesmann, Broag, MHS etc do.
All the pipework is commercial the Op is looking at copper a fraction of the size, it's only a bleeding bungalow not a 10 room mansion.
If I was incorporating Solar, ground source heating etc, then the buffer tank is 100% the way to go, in a small 3-4 bed property that only need one boiler, if it was installed properly it is over the top and expansive.
Low loss header may be crude but let me assure you, simple works and works very well irrespective of size, you are trying to over complicate a small domestic system.
The HX are going down because of the poor circulation, what you need to do is read and understand what I have said then go away and think about it, especially the bit that say the larger pump can stall the smaller pumps when they are modulated down to the lower speed.
You can argue all you like, I don't know you or what you do for a living, and don't care for that matter, but at least listen to your betters.
And I don't give a monkey's what or how the Germans doitall, or care either, they are not the worlds authority on good design,