OK,
iphone 5g screen damage, juice / booze had leaked in upper body through crack. Phone seemed to work before repair although lcd was dead. Power button was 90% gone but still managed to shut off with home/power.
Repair completed, new screen testing = no start, no sign of anything on screen. No sign of battery low, NADA. When plugged in charger, double vibrates every 10 seconds or so. There was corrosion on lcd connectors (logic board connector) so I assumed bad connection. Reposition them 3 times, same old. 4th time the double vibration got replaced with a mild beep (?!?) one only on connection of charger (still nothing on screen). Throughout the whole process, phone overheats even while charger is disconnected and the phone is OFF.
Battery change = problem solved.
Long story short, lesson learned - try the simple solution first!
Thanks for the thread, really resourceful!