If your PEX lines into your house are 1", I would get manifolds with 1" bodies so you could close down all of your heating "zones" and still flow the entire amount of water through the manifold with no ill effects.
The reason I went with a supply and return header was for simplicity. Put your supply header on the end of your supply line somewhere convenient in your basement or such. Put your return header on the beginning of your return line and somewhere convenient near your supply header. Connect the two headers with whatever you're heating (for mine I have my water/air exchanger, DHW, and radiator in the basement). The heat loads receive the hot water from the supply header and return the cooled water to the return header. This only requires ONE circulator. If you use a single large manifold with all zones supply and return off of that, you'll need one circulator to circulate the main loop and a small circulator on each loop to circulate water to/from that loop.
If you do one continuous loop for your whole system, it could cause heating problems. If you did that with your furnace, flatplate, and garage designed around a 180 degree initial water temp, the temps would drop for each item as you go through it. Let's say your furnace was setup for a 20 degree delta T. 180 degrees into your furnace, 160 out. That means 160 degree water into your heat exchanger and maybe 150 out. That leaves you with 150 degree water to heat your garage and maybe 130 degree water returning to your OWB. That means your OWB has to heat the water load 50 degrees before it can return to heat your house properly. With a manifold system, your furnace would draw 180 degree water and return it 160, your flatplate would draw 180 degree and return it 170ish, and your garage would draw 180 and return it 160. Your OWB only has to raise the temp 20 degrees then. It would be a larger volume of water to raise 20 degrees, but it's all about the delta T and longer burn times as opposed to short inefficient fires.