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The keypad errors **might** be from the abnormally low battery.
**might**
All KT(*) devices are IR scanner rather than capacitive touch screens (note the extra thickness of the bezel).
So do an extra careful job of cleaning the bezel edge where it meets the display surface. Those keypad system errors might from the scanner not being able to "see" through the dirt.
A **good** (4.2 volt) Li-Ion battery should never go that low - it has been over discharged too many times and for too long.
Buy a new one.
Then fix the charging problem, which you most likely have already found - broken solder joins between connector and motherboard.
(PS: Don't leave the new battery in the device except long enough to make tests of repair efforts, you don't want to over discharge it until you get the charging circuit working again.)
Ask google or other web-search tool about re-flowing possible cracked or broken SMD connections. You should find the info under "SMD board rework" topic.
Be sure to look up the temperature profile to use.
Since the connector is at the very edge of the motherboard, you should be able to use make-shift tools to re-flow the solder joints.
(Ah, battery removed from system, motherboard removed and by itself, metal foil used as a heat shield/hot-air deflector away from all parts other than the connector, ... (you get the idea - hot air to connector/motherboard area ONLY). )
Of course, if you have access to an SMD board re-work station, all the better.
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