As I said earlier, I am not an expert in the discipline of philosophy, and I haven’t read Kant yet - however, I do speak fluent German, so what this term (which translates roughly as ‘thing-in-itself’) refers to would be external reality before it undergoes processing by the sensory apparatus and the mind. So it is meant to be that which exists independently of any perceptual process, the true essence of reality, unsullied by mental constructs, so to speak.
The trouble is that we have no way of accessing that external reality, except through sensory and perceptual processes. Anything you are aware and conscious of is a construct of your own mind - it is at best some representation of the ‘ding-an-sich’, but a representation that will be more or less incomplete and distorted in ways that are not necessarily readily apparent. So it may very well be that there is an underlying external reality of some kind (though I would think one could make a valid case questioning this), but we will never be able to access it directly, all we can ever know is our mind’s representation of that reality. It’s much like being stuck having to look at a map, without ever being able to go to the actual territory.
In what sense would a single elementary particle be a ‘chair’? How many particles does it take before the ensemble acquires ‘chair-ness’?
No, QM does not allow us to conclude that there are no elementary particles (or quantum fields, whichever way you wish to look at it). It does, however, make it clear that the dynamics of those constituents cannot be classical, due to the existence of observables that don’t commute, and due to statistical correlations between measurement outcomes that are stronger than classically allowed (entanglement).
I’m not sure what you mean by this.