As far as I am aware there was a fixed window for buying the PSL for your own seat that shut a few years ago, so there isn't an option to buy one for your own seat currently.
Not withdrawn as such, more that there was always only a limited window to buy them from the club.
You can still get them if you can ever track down an individual selling one.
No - if the cunning fan was going to every game they'd simply get an adult ST.
But if they knew they'd only go to ( for example ) 12 games, then it maybe that a very cheap child ST + 12 upgrades is a cheaper way of going to those 12 games AND SITTING WITH THEIR CRONIES than buying an adult ST.