A circulating pump will not restrict the gravity flow. It acts as a circulator only. The back boiler on the stove should have a pump on the return pipework wired through a pipe thermostat located as close as possible to the stove itself and located on the flow pipe. The non-return valve should be located on the positive side of the both circulating pumps.
You do not have to "knock off" the oil boiler when then stove reaches temperature as the oil boiler will be thermostatically controlled anyhow. You should however install a room thermostat on the heating & hw zones that will turn off the oil boiler.
Using the 4 connections is not more efficient but can be advantageous for separating the dhw circuit from the radiator circuit. It is also ok to use 2 and then split.
The circulating pump on your oil boiler should then be on the flow pipe so that it is not pulling against the stove pump. Make sure that the osvp is plumbed behind the boiler pump so that it does not pitch water through the osvp. Ideally pipework should exit flow pipe from boiler, then 3 bar safety valve, then 1st tee is osvp, then no more than 150mm away the f & e pipe, then the circulating pump, then the aav and then tees for heating zone and dhw zone. However, in a situation where there is more than one boiler, all circuits should have a route to the f & e pipe, so it should be placed on the return in a location where this can be achieved.
The cylinder in most cases cannot go into the attic as the cwst needs to be above it and usually there is not enough space. If you are going to install solar at a later date, then it is more cost effect to change to a triple coil cylinder now. Copper is ok for this, but if your budget can stretch to stainless steel it is better. Be wary of some of them. One particular Irish manufacturer only offers a 1 year warranty on their stainless steel ones and they nearly all leak after this time. Joule or Warmflow triple coil cylinders are pretty good. They also have good warranty periods. I think 20 years.