There is no single deciding factor. It's weighing the State's evidence against your available defenses. The most important pieces of evidence in a DWI are the breath or blood score and the video(s). After you have reviewed all of the discovery with your attorney, you will be able to evaluate the DA's offer and decide whether accepting it outweighs the risk of going to trial.
That's a sophisticated question. There is no statute on this subject. There is a little caselaw, but nothing that sets out a bright line rule. It is an argument I still make when I have the right set of facts. Whether or not I win these hearings also depends largely on which judge you have.
Signing a 12.44 does not make a case aggravated. It is either aggravated by the facts of the case or it's not. A DWI Child is not aggravated for the purposes of sentencing. How this will impact his immigration status is a question for an immigration attorney.
Your attorney is in the best position to answer this question. Just because the return isn't showing on the system yet doesn't mean the witness hasn't been served. If you are unable to execute service, you may or may not be granted a continuance. It's at the judge's discretion.
There is no average. You might find one attorney who would do it for a thousand and another who would charge five. Just call a few ticket attorneys in your area. They won't charge you to discuss the facts and give you an estimate.
That is a jury instruction you have to ask for at trial. The judge may or may not allow it, depending on how the evidence develops during the trial. I think your defense is one even the DA will find sympathetic. Hire an experienced defense attorney, and I have a feeling you will reach a resolution that avoids trial.