Total PSI Created from a 3/4 HP Sump Pump in 5/8" Pex Tubing

It would probably help to manifold the PEX tubing into four parallel runs of 50 feet each. This will better match the flow of the pump. You could also get by with less HP.

Yep.

Like Lyn sez, yep. Since the water is flowing back to the source, a lot of energy is recovered. Use a large pipe from the pump to the manifold, have throttling valves on each fifty foot run so they all get the same amount of water.

Solar hot water heaters have very small pumps. You don't want the water going too fast, it won't have time to get hot.

If the runs are the same length/configuration, they will naturally balance to equal flow without needing valves.

The operating point in the pressure/flowrate plane is where the pump performance characteristic curve and the system characteristic curve intersect. Not knowing the shape of either curve, it is impossible for the forum to determine the full answer to this question.

Overpressure downstream of a pump can be prevented by using appropriately-sized pressure relief valves, however it is usual for the downstream piping to be strong enough to withstand the dead-head pressure of a pump, thereby making the provision of pressure relief unnecessary.

Apart from all of the other relevant answers, it would want to be a really small swimming pool for you to gain any benefit from such a small collector.

It is generally acknowledged that a swimming pool solar collector needs to be roughly equivalent in area to at least 60% of the pool surface area (dependant on local insolation, wind loads, pool cover etc.).

I would also suggest using a smaller diameter pipe in multiple parallel runs, as it is the surface of water exposed to the sun which does the heating, and more, smaller pipes will expose more water for greater benefit without reducing the flow rate too much.

Having many sump pumps in my time I cannot imagine one that can create enough flow with restriction to achieve 75 PSI.

I have considered a similar plan except to use a manifold with up to 250 feet each of low pressure black irrigation line which is available in 1000 foot rolls and can weather the sun very well. Also I would use a white reflective surface in hopes of getting more reflected light onto the backside of the black plastic for improved heat gains. However after thinking about dealing with that much plastic and the likely kinking problems I have yet to try. It is however very cheap to buy about $150 US dollars/1000 ft.

Beware the maximum temperature inside a solar collector can get above 100C (212F), and thus would be too hot for the PEX to retain its strength. I would choose copper, and paint it black, then cover with a layer of glass. Even in Scotland this damaged connection pipes made from domestic hot water feed pipes for a washing machine (rated for 95C).