Luxury apartments do reduce the housing shortage. Cashed up boomers can move out of the nice family home they've been living in since the kids left and upgrade/downsize.
Building the high end and having people move up helps with the high cost of construction also.
The general phenomenon is well known to those who study this kind of thing, but yep, building housing of any price helps reduce costs overall:
Depends on if people live in it or buy as an investment and leave it vacant or air bb as often happens in London
Not really since those people would otherwise some different existing property to do the same thing with.