If I do as you suggest that would allow me to run glycol in the slab , just in case....
Is that why you suggest this option over using a mixing valve?
Multitude of reasons, but one of them is to add glycol, yes. You'll still need a mixing valve to temper water/glycol to the slab, as 160-180 or whatever temp your boiler water is is too high for a slab both for concrete longevity and efficiency. Too hot of water wastes fuel as well as heats the slab too fast which often overshoots the room temp excessively after the air temp reaches the setpoint but the floor is still 100+ degrees. This is also very uncomfortable on bare feet. Slabs should be mixed down to 100ish degrees ideally. Another reason I recommend exchangers is corrosion and sediment. Floors are often the low point in a system, and any sediment will settle to the bottom, eventually plugging the floor loops. This is why they recommend oxygen barrier PEX for slabs, because any oxidation (rust) from pump impellers or any ferrous piping, fittings, sediment in your domestic water, etc will do the same in an atmospheric system such as an outdoor boiler. Premixed glycol is inhibited, as well as diluted with deionized water which has absolutely no mineral or sediment in it to settle to the bottom. When converted to a closed system, the whole system is now completely isolated from oxygen and therefore cannot oxidize, allowing ferrous material if you want it. Fluid transfer is also much easier on the pump under pressure vs atmospheric.