Yes BillS I am sure that the belt driven engine water pump would be strong enough to flow through your slab pex. Think about it and the pump when the thermostat opens has to be able to massive flow through the radiator core.
Look at heater return line on all of your current vehicles and equipment's.
Return lines NEVER into the lower radiator. Always into the lower block/water pump radiator return. Look and there even may be a plug down there.
Take the fellows too hot for pex seriously. Plastic and pex failures with neighbors and families systems why I have insisted on old style sweated jointed copper piping in all of my systems. Learning to sweat/solder copper piping is actually fun. Brazing metals is fun too. Stick and MIG welding is either work or blinding boring. TIG is back to two hands and one foot challenging fun again.
Anything you add for control beyond just a manual flow valve will get complicated to design; WILL sooner or later have system failures requiring pin-point diagnosis and specialty parts replacing.
I do not know the force water furnace flow control valves they are referring to.
What you choose to use DO invest in a spare set-aside components!!
Pretend if you do not have it on-hand, or could make it, that it does not exist.
You raising chickens now and freezing them to NOT have to be dependent on the drive-to, meat market counters.
Imho, DIY energy making should be the same.
Especially if you are out Rural, off-grid.
Keep you systems, simple, robust and easy to repair. Make no hesitation sacrificing the great gods of idealism's, and perfectionism's to achieve this.
Gasoline chainsaws are wonderful. Electric chainsaws are good too. Swede/bow-saw backs them up.
But Always Have A Good Sharp Worked Proven AXE.