Iphone X Charging with 12W ipad charger

no, more than 5volts toast the device

current are drawn not pushed so the ios device will draw no more current

then it's build in charging system was made to draw

watt == volt * current

and watt and current are in terms of psu's a max amount devices are able to draw before lowering the volts

where a lowering to 0volt would mean a short circuit

yes