My main problem is that when I am taking a shower, I lose all hot water when someone turns on cold elsewhere. I have been led to believe this is because of an insufficient water supply to the water heater, leading to a loss of pressure on the input and thereby decreasing the pressure on the output. I believe I could fix this by bringing in a 3/4" or 1" line first to the water heater via PEX manifold, and then do PEX home runs to all the fixtures in the house.
I have attached pictures below of my setup, and I welcome any input.
(Originally I had pretty much the entire flow backwards. I have updated my images accordingly.)
I have a 1/2" line D that feeds both the boiler and the water heater E. F comes out of the water heater and reduces to a 1/2" line to feed the house fixtures.
I used the installation instructions for the 9DM3 backflow preventer pictured, and the following is an installation diagram. I have both the 9DM3 and the 1156F pictured between D and C, and I am confident this part of my home matches the installation instructions.