Re: The schematic link: The injector tee to my view adds restriction to the primary gravity circuit and not as they claim encourages. Injector tees are similar to TRV's in they open and close with a thermostatically controlled capillary tube. I would not install one on a primary circuit for solid fuel.
The schematic on page 3 is nonsense. They have an unvented oil boiler going into an open vented buffer tank. No harm in that but why unvented? They also have an unvented pellet boiler but that has no expansion vessel, etc. and it is only going through a coil in the buffer. It is also a very busy buffer tank with so many coils. Not much room left for water! All links from the document are broken or non-existent.
Basically, keep it simple. Heat will always try to find the easiest route to circulate so design the system with easy uncomplicated no fuss to encourage this and it will work.
With regard to positive pressure, if you think of everything in front of the pump is positive and everything behind is negative. The neutral point is the point where they meet. The neutral point should be kept as short as possible. It is hard to explain without a diagram, if the pump is on the flow the first tee from the boiler should be the osvp, then within maximum 150mm the f & e and then the pump. This means that all the system is ahead of the pump so it will be in + pressure and from the osvp to the f & e will be neutral and from the f & e to the pump will be - pressure.
If the pump is on the return then all the system will be negative. Nothing wrong with it and it will work fine, but all joints are on suck which microscopic air can enter. Generally, the pump on solid fuel will be on the return as they higher temperatures from solid fuel can damage the plastic impellor in the pump. Then the second pump would go on the flow. If both are on the return, they could pull against each other. This is where you find some people with dual systems find they cannot have the oil on at the same time as the solid fuel. They find pipes banging, etc. and this is usually caused by pumps pulling against each other and the heat can go nowhere.
If you pm me with your email address and an outline of what you have and what you are trying to achieve, I can do up a diagram for you.
BTW, I would not opt for LPG.