Atypically, the Michigan law is similar in several respects to Arizona's. Juvenile records are not automatically sealed. YOu would have to apply to have the matter set aside. That's the term Michigan uses for an expungement. It's along the lines of a dismissal.
Unfortunately, as you've already learned from the Public defender's office, Michigan will only allow the first of your convictions to be set aside, so it isn't particularly helpful to you as you've got two. You can see the expungement law of your state here. (see link)
Here's some information on the Pardon process in Michigan. (See link)
Pardons are a long shot but you've been out of trouble for a long time and that counts in your favor.
The Lawyer Referral Service
for the state of Michigan is here and an expungment/pardon attorney can give you an indication of whether the likelihood of being able to get the governor to grant your request is going to be worth the money.
One other probably expensive longshot would be to hire a criminal attorney to reopen your own old case and petition to get the charge dismissed in the interest of justice
This would not be supported by the penal and procedural laws in your state. However, a judge always has power under Equity to grant a petition like that even when the law doesn't provide for it, if it is necessary to prevent an injustice from occurring. Courts
are sparing in their use of the equity power and many judges will not touch this at all, but it is something you could explore with a local criminal lawyer who would be able to tell you how viable this would be with your particular judge and whether it would be worth the money you'd have to lay out to get it done.
I wish you luck.