without more information its hard to say, but heres some general suggestions, pull the battery, there is likely still water or corrosion inside witch may cause more damage with power going through the circuit, if your conformable with it i suggest taking the phone apart and cleaning the board with isopropyl alcohol.
it would defiantly help, it sounds like the contacts are getting shorted out, i would recommend a higher percent alcohol like isopropyl witch is 99% + alcohol, drys better as it has less water than standard rubbing alcohol, if that doesn't fix it you could try a new charger port
since the computer does see the sdd and the ssd has lost half its capacity and the computer still works otherwise, the problem should be the ssd itself
check the home button connector and the charger port flex slot it plugs into for corrosion, and cleaning that with isopropyl alcohol, if that doesent work next step would be new charger port , final step is new home button, but a new home button will lose touchid function no matter what
sadly no, all in one computers like the imac are as upgradable as a laptop, as in not upgradable because to get them as thin and small as they are each part is shaped to fit that one model