What can I do if I was ordered by a judge to do community service, but when signing up found out I was banned from the program?

Go back to court and tell the judge about this.

Check the CDCR inmate locator online.

Will I ever be able to purchase a gun with misdemeanor domestic violence charge?

Nope. No guns for you. Ever.

In sentencing can I insist my regular criminal judge be there and not a replacement judge who is unfamiliar with my case?

Yes, unless you waived that right.

Doesn’t mean much to be honest but it might help a little.

Fascinating. QUIT STEALING.

With a misdemeanor conviction for 417(a)(2) brandishing, is there a ten year ban on firearm ownership? If so, any way around it?

There’s no way around the ban. 1203.4 won’t help either.

Can somebody be convicted of a serious crime solely on the strength of their own confession without other evidence?

Sounds like a homework assignment.

If cops, during course of conducting a search warrant, see something else illegal, are they allowed to get another SW?

Yes. They could get another warrant or just seize the illegal item.

Stupid mistake indeed! Hire a lawyer.