1. Do you think this is enough to establish unrestricted right should that become an issue? To clarify, the demand letter does not specifically say that I brought up the overpayment on my own.
Doesn't matter. Keep both the demand letter and the cancelled check for at least 3 years (though I recommend 5).
2. Do you think I could/should request any documentation which can establish unrestricted right?
Nope. You have all you need with the demand letter and cancelled check.
The total refund here is above $22,000
Shouldn't be an issue, so long as you have taxable income in excess of that amount, from which to deduct the amount you paid back. Otherwise, the deduction may not help that much. I don't know how it works if say for example, you only have $10,000 of taxable income *before* the right of claim. While I've never heard details or experienced an amount that large, I would fully expect you to be audited by the IRS on this. But this should not be an issue.
Generally and audit of this type is what is referred to as a "paper audit" where the IRS sends you a letter asking for proof. The demand letter and cancelled check are all the proof you need. You send a copy of each to the IRS and the issue "goes away".
