Repeating it doesn't change the answers.
1) you were legally an adult when you became disabled. It has NOTHING to do with when your grandfather passed. If you were not disabled as a minor then as Kivi said you have no claim to the benefits as a grandchild if you were not disabled at least two weeks prior to your 18th birthday AND the death was directly related to your grandmother's disability.
2) While one branch of the federal system may have realized your grandmother passed it does not mean the rest did and that she was entitled to the benefits. You are going to have to contact them and find out WHY they are saying the money needs to be paid back. What you fail to realize is that if you ignore this it can impact other federal funds you may be entitled to including your tax refunds.
CONTACT THEM and find out what the reason(s) are for the denial and request two years later. The last thing you want them doing is determining that you FRAUDULENTLY got the money. If you contact them and they actually are due the money back that is your chance to arrange a payment plan. If you ignore it they may force less than desirable solutions on you.
No matter what the odds on you getting these payments reinstated to you are zero. You were legally an adult when disabled and therefore under the FERS system are not entitled to the benefits.
"That's just my opinion, then again I might be wrong." Dennis Miller