Image 1 - A8 SOC and Image 2 - Main PM IC on logic board. i guess heat produce from the PM IC.
both chips were located at very closed upper and lower sides of the logic board hence you feel A8 get hot. Main PM IC doesn't drive RF directly as ordinary phone so checking on RF AMP/drives short circuit doesn't make sense to this case.
(Question )Multi meter - Multimeter probes produce narrow current to measure components on ohms range but it never enough to fry your motherboard components and its not enough to do things that statistic electricity of your body to damage hi e-sensitive components on logic board.
9V battery is use to power up the multimetter and it will not produce 9V though the probes for any case.
Reason the occurring these kind faults on apple devices
- if you use 3rd party / apple not certified charger to charge your device
- if the phone damaged by liquid
- Low quality wires connected externally (Damaged) lightning cable
- SOC, NAND Vbus 1.3V-1.8V short circuit
Solution : This fault is every complicated to rectify and you may required hi-tech tool to monitor signaling.
but can be recommended you to replace Main PM chip : (part number - Apple/Dialog 338S1251) it required high level BGA reworking skills to replace the chip. (Not expensive chip :-) you can order via eBay or amazon )
PM chip replacement was hard but some moved capacitor fixing was harder than Chip replacement. anyway fault is repairable.