Micheal

if you haven't already i would try cleaning the dust out, make sure the fan is still spinning, it sounds like a possible overheating issue

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 is a lot of work swaping every single part, but yes, you can replace the housing

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

well the mute switch is on the same cable as the vol up/down, so i would start with replacing that

in the iPad mini the flex cable is soldered on.

did you transfer the tiny flex around the headphone jack? it is related to preventing interference in the jack

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