Since it seems the toxicology will come back showing alcohol in your blood, you should hire a lawyer immediately. You may have a defense based on consumption after the accident if you have witnesses who can testify. This investigation should be done by a professional and as soon as possible. You may also have a defense to admitting the results of the toxicology on procedural grounds. This remains to be seen. You are going to have a hard time with the leaving the scene of an accident, as well....
Held liable for for what? The biggest risk is the denial of a the green card and possibly an accusation of marriage fraud. You are making a HUGE mistake if you go through with the interview without consulting with a lawyer. You need to give the lawyer time to prepare, as well, so do not wait until right before the interview. Find someone ASAP.
Yes. Once you complete pti the court should issue a dismissal order. The charges and any plea are dismissed. Six months from the date of that order you can expunge the records so the record of the arrest is gone from your criminal history.
Why can't you get a public defender? This is too important to do on your own, and if you can't afford the transcripts, it sounds like you should qualify for a PD. Doing your best with the brief is probably not going to be good enough.
If you were driving while suspended for a DWI, not only will getting restored not solve your problem, you are facing mandatory jail time. Cops are not lawyers; they shouldn't give legal advice and you shouldn't listen to it.