The guardian should be present and sign the documents.
A person charged with a crime is the accused or the defendant.
no
guardian
The accused, or the defendant.
Another person can sign legal documents for you only if you execute a Power of Attorney or if they are a court appointed fiduciary such as a guardian or conservator.
A DEFENDANT - In criminal cases is the person accused/charged with the crime.
indictment
Autism is a very wide spectra so how a person is varies greatly. If the person is a minor you could be charged with kidnapping unless you have consent from his parents or guardian. If he is an adult and has a guardian you will also need consent. If he is an adult like the rest of us and do not need a guardian you do not have to tell anyone, the choice to go with you is his. If you forced him to go you would be charged with kidnapping.
In the US, yes. To even be charged, the accused must be arraigned and appear (or waive appearance) in a preliminary hearing.
A jury is what determines if you are guilty, after being presented with evidence and hearing what the person accused is being charged for.
Replace the word informed with "charged". It varies on a state-by-state basis.
Yes, that would be illegal. The person impersonating your daughter's guardian can be charged. Tattoo shops are supposed to get a legal affidavit signed at a notary to prove the person accompanying your daughter is her legal guardian/parent. If the tattoo parlor failed to do this, or overlooked it, then they too can be in legal trouble.