You don't need to full quote my post just to ask a further question.
If it was so easy as you seem to hope all the people out there would grap a cheap live demo unit and just exchange the motherboard from an old or cheap phone ... [emoji6]
You not seem to know how smartphones are built. They are highly integrated devices with just some ICs. The motherboard or the mainbord IS the device. There are only very little extra parts (housing, display, speakers, microphone) that are separated from the mainbord. So even if the board of an other (samsung) phone would fit mechanically (what it doesn't) then you would have an (let's say as an example) Samsung Galaxy SII in a S5 look. There would be more difficulties, for example the screen would have the wrong resolution and so on.
So the only mainboard you could use is from a Galaxy S5 phone, which one exactly I don't know.
You can see your S5 LDU as a spare part for a broken S5 or a broken S5 as a spare part for the LDU - from two make one.
In the end there is no easy or cheap way to use the LDU as a phone and that's what samsung wanted them to be: a demo unit.