Low loss header off of buffer tank?

A buffer tank, if piped correctly IS a low loss header and there is no need or want for a second one. Can you share some photos? What you've typed out with no punctuation is very difficult to understand.

For any arrangement of piping where there is one circulation loop — in this case, your main line from the mechanical space to the house and back — and several other loops pulling off of it and returning to it, with a way for the main flow to bypass the other loops, it's all a matter of relative flow rates.

One needs to consider primarily that bypass, and the question always is — which way is the flow going in the bypass? If your main loop has a relatively high flow rate, and the total of all the secondary loops pulling off it is less than the main loop, then the flow in the bypass will be from the secondary loop takeoffs towards the secondary loop returns, and on back to the source. On the other hand, if your secondary loops in some combination or combinations pull more flow than the main is delivering, then the flow will be the other way, and the secondary loop water will be a mix of lower temperature return water and higher temperature main water.

Fundametally it doesn't matter whether it's a low loss header with all the secondaries coming out and coming back or a simple single manifold pipe — so long as there is a way for main loop water to bypass the secondaries, you will have this hydraulically.

So the question I would ask is what is the flow rate, in gpm, in that big main loop, and what are the flow rates in your secondary loops (velocity is irrelevant here — what you need is gallons per minute).

Will those pumps pull off the buffer tank? hmm. Well, that would depend on the head loss through that long pipe and the relative elevation of the buffer tank and the rest of the system, and whether or not it was pressurized. The key factor will be what is the absolute pressure at the pump inlets when they are all running. That must be at least atmospheric pressure, or you would risk cavitation — which will ruin a pump in a hurry. There is also the factor of the total head they have to pump against from the various losses in the piping.

One thing if you were to do that, you would want a supply manifold and a return manifold at the house — and no bypass. If you wanted to temper one particular loop, you could do that, with the tempering valve pulling from the supply header and the loop return with the tempering valve on the suction side of that pump, and the excess return going to the return manifold.

so from the buffer to the pump manifold

250' of 2" pex flowing 25 gpm puts you right at 4 fps velocity, so that would be a good flow rate to target.

Now you need a pump capable of 25 gpm @ 7' head A high flow, low head circ something like this Grundfos 43-100

If in fact you need 25 gpm at the zone pumps?

How do you come up with 1415 PSIG

Water exerts just less that 1/2 psi for every foot in height, .433 really

This is true at the bottom of a lake or the bottom of a tea cup. The pressure on your components is just about the amount of water height above them. Nothing at all to do with the volume of water or the weight of a gallon of water.

Vapor pressure is the minimum amount of pressure that must be applied to the liquids surface to prevent it from boiling.

With that pump up high on the tank you may not have any pressure. Move the pump to the bottom of the tank, just as it is on the OWF, and I predict your troubles go away.The water level in the OWF is the highest elevation point in the system. Else the water would spill out the top vent opening.

If you are interested in the numbers, here is how OWF systems go negative pressure.