Patrick, I think you are clutching at straws here. For keyboards, there is a section where the specific key is detected. Typically it's a combination of a pair of lines being activated when the key is closed. So, for example, to make a "a" key press, the switch closing at "a" closes two circuits, one to put it on the row with the other keys (sdfghjkl, etc), and one to be the "column," maybe the "qaz" or more options. Other circuits the look for special keys (shift, alt, option, control, fn) to see if the intersection of the key is to be modified by the special key. At the end of that bit of physical logic, we have a code saying, "This key was pressed along with this one." So now we know what key. That same set of keys on an external keyboard result in the same signal. That signal now is processed by a logic circuit to say, "Given that set of keys, and the keyboard selection set for me, the intent was to make an "a."
What I suspect has happened is that the first part, selecting the key(s) is probably working, but the second, "what did that key combo mean" is not functioning. It is receiving the same signal from the internal keyboard and the external one, but isn't processing the results. And that failure to process points to a chip or circuit on the logic board being defective. Probably because of the liquid that was spilled.
If it were just the keys, the external keyboard would "fix" the problem. It didn't, and the errors as reported on the external keyboard being the same as the internal, points to something downstream of the simple key-sensing circuits.
That said, the OP can certainly try another keyboard, but given what has been reported so far, it will most likely have exactly the same symptoms.
