All of your income is considered when receiving federal SS benefits. So it is your total earnings they take into consideration not a basse period. So it down't matter what you were making when you applied. State disability programs use a base period but not the federal government.
As for getting your onset date readjusted that's a tough one. Obviously, you would need evidence to prove the earlier onset date. It wouldn't change your monthly rate but it would change your back pay benefits. If you were earning more than about $680 a month 8 eight years ago (during the year before your applied for onset date) you wouldn't be eligible anyway.
Call Social Security and ask them. I would be curious as to what they say.