Since you have power of attorney, you should be able to obtain a copy of the PSIR from the lawyer. If they do not want to share, have your son sign a paper saying they can share the file with you, or have him request a copy of the file. The PSIR is given to the judge by the probation department - not by the lawyers. There is supposed to be a drug & mental health evaluation included unless waived (although I find that those items are not usually done - at least in Houston - unless specifically requested.)
The document is file stamped once. If it is in the court's file (which you can check by going to the clerk's office with your son's name & DOB & asking to look at the file), you can get a certified copy. The clerk's office, for a charge, will provide the copy & will certifie it - usually on th back of the last page. You can ask that each page be certified ifthat is what you want.
Mental illness does not excuse illegal behavior unless it rises to the point of insanity. In fact, frequently, a mental illness is a double edged sword - meaning that it does as much damage as good to tell the judge that the client has some mental illness. If your son was on medication at the time of the offense & committed it any way, what assurance will a court have that he won't do it, or something else, again?
There may be a record of the plea, too. Call the court reporter & ask. You will have to pay her/him to type it up. If the PSIR was offered into evidence, you can get a copy from the court reporter, too.
An attorney can get the presentence report if it is that important to you. As for your questions, there may be a file endorsed stamp on the first page of the report but not on every page thereafter. It is a little paranoid to think that the "dream team" would slip something into the report you got that the judge never saw. As to whether the judge is aware of your son's mental problems you can't know without getting a copy of the report. I strongly suggest you get a TX lawyer to help you with sentencing issues.