You haven't proven that it can't be proven that 2 + 2 = 5. All you have proven is that if 2 + 2 = 5, then mathematics are inconsistent. Which they could be; you can't just assume math and logic are consistent.
Well, you were responding to someone who was using the correct formal terms. You, after all, don't dispute the second theorem - but given it uses the term "consistent" you can hardly complain when someone makes their argument using the same terminology as the theorem you agree with!
Interested in what you define "consistent" and "valid" as in laymans terms though.
Assume it can be proven that 2 + 2 = 5.
Then 2 + 2 = 5.
It can be proven that 2 + 2 = 4.
The closure axiom of addition states that the expression 2 + 2 is a unique number.
4 != 5.
Therefore it can't be proven that 2 + 2 = 5.
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I'm sure it can't be that simple, but logically I can't see a reason why not.