You've posted lot of information, but many confussions.
Also noted wrong assumption; suposing 18.5V means the voltage expected by you from charger.
Charger voltage depends by its type; is marked on it.
Please, stop to post SUPPOSED voltages; we need accurate information.
Seems that you get 3v3_S5/SUS but no other voltages; confirm if correct.
Trigger SMC_ONOFF_L and check for drop pulse at PM_PWRBTN_L.
Also noted wrong assumption; suposing 18.5V means the voltage expected by you from charger.
Charger voltage depends by its type; is marked on it.
Please, stop to post SUPPOSED voltages; we need accurate information.
Seems that you get 3v3_S5/SUS but no other voltages; confirm if correct.
Trigger SMC_ONOFF_L and check for drop pulse at PM_PWRBTN_L.