Apart from a big dose of jealousy you mean? Wow, just wow! You have years of fun locked into that lot!
Until very recently (my current furniture project in fact) everything I have built has been from reclaimed timber, some of it firewood. You could say this is my “thing” so I apologise if this looks a bit like self-promotion - I just love not paying for timber!
I agree with @Tokoloshe that chemical or animal stains are easy to spot generally and are pretty innocuous, and you have enough to discard any that looks dubious. And think beyond chopping boards! You can always laminate it to make wondrous things, or even make veneers to bend it.
HOWEVER depending on where it comes from and the age of the flooring, in the EU I don’t know, in the US there MAY be residues of antisap stains which to be completely alarmist once contained compounds of mercury with chlorophenates (pentachlorophenol), or if you live in Australia even DDT from insect sprays (because we used to bathe in the stuff).
In all probability, the traces of those things would be minute, and you will be milling off the treated surfaces and mostly using some sort of finish as well to encapsulate them. If you are really worried, give one to your mother-in-law as a test! 
DO wear a proper mask while machining (I prefer a full respirator) - there’s just a tiny possibility that breathing the dust could be even more hazardous than usual.
Then enjoy the stuff - cut off the tongue and grooves as you unload so you can stack it, I usually run a quick “nail finder” cut with a hand planer and cheap replaceable blades to get rid of most of the paint and stain before storing and then plane to size for each job.
I have just about run out of a 1 cubic metre load of flooring I bought twenty years ago - It’s the dark red stuff in the pics - the work bench on my compressor cupboard is entirely 40mm strips of the stuff laminated on edge. You will find ways of disguising, hiding or featuring nail holes depending on your mood.
One of my workbenches - even the MDF was reclaimed.
My generator cupboard - laminated hardwood flooring top.
Chairs (6 off) for each of the grandchildren, from an old pergola with “that” flooring as seat and back.
Step stools from wood rescued from a bonfire.
More flooring at use as clothes line tensioners.
Bench from a neighbour’s garage demolition
Boat from another neighbour’s piano.
One of my print-tables - printed frame with shelves of ancient pine flooring (lightly sanded and waxed)
As far as age, it came from a house in a sub-division that can’t be more than 20 years old. I will maybe try that mother-in-law test you mentioned though, you know, just to be sure, in the interest of safety 
