SYS rail can be found at the “2R2” coil next to the BQ IC, to test just put one probe on ground and the other on one side of the coil (doesn’t matter which side)
EMMC causing the low reading is a concern, inspect the few passives on the module and see if anything stands out, or better yet post a pic of the EMMC module either side so we might inspect it.
If it isn’t one of the passives at fault then next thought might be liquid under the IC given that this is quite a high short, and keeps changing, give the IC on this module a flux boil (unless you suspect sugary soft drink was the cause) and try flushing any corrosion out from below, then rinse with alcohol and repeat heat, don’t bring the IC itself up to reflow temps, just enough to activate the flux - so approx 200C should be good.
Don’t waste your time taking readings on the EMMC module itself, it is the cause of your 1V8PDR short. so we already know it’s the cause of the fault on this rail.
Refrain from powering the console on further until the fault is resolved less the EMMC be corrupted