It's not uncommon for an innocent person to plead guilty to a misdemeanor that comes without prison time, in order to avoid the risk of being convicted of something worse with years in prison. Especially when they have multiple accusers, and don't have hard evidence to exonerate themselves.
You don't need hard evidence to exonerate yourself ( you don't have to prove non-existence). It is the accusing party that needs the hard evidence to convict you of crime.
While many unwitting people can be victimized by desire to avoid intimidating experience of trial and criminal proceedings, for a man who had honorably served his country in war it would be unbecoming to be intimidated by its' justice system.
How do you intimidate a guy who flew F-22's over the enemy line of anti-aircraft fire with the hassle of sitting in the court room and defending himself of false charges?
I find it hard to imagine.
Nor do I think he is a man of low intelligence (you can't get into Air Force with low aptitude test results. You may qualify for infantry but never AF).
Therefore, I doubt this as a likely reason for the guilty plea as well.
If he is innocent and did plea guilty,then it would most likely be due to his unwillingness to bother with the any hassle than being intimidated of standing a trial.
Now, especially if innocent, he will learn a handy lesson how wrong he was.
But, again, I somewhat doubt his total innocence. I mean I am open to consider that he may have had no ill intentions ,acted in good faith and wanted to do his best for all involved while his uncles were bad guys who decided to sacrifice him to get their money back, however, it doesn't negate the fact that he engaged in trading of securities without passing required exam and getting registration. Surely he didn't just sit there and watch as his uncles traded his funds and later accepted a guilty plea for what they did and accused him of doing? Unfortunately for OP, this is one of the strictly regulated trades with severe (as evident from his story) consequences for those who are found in breach.
You are conveniently ignoring 2 important things that I previously mentioned: (1) if he actually didn't do anything illegal, the written contract could have shown that his uncles were the wrongdoers and not him. In such a situation, the contract would have hurt the uncles, so the contract isn't going to absolve wrongdoers of their guilt; the contract is to help the innocent to prove the truth. And (2) a lawyer should be consulted before pooling nontrivial funds with anybody for anything (in a situation like this, the lawyer would either advise against the arrangement or direct them to get the appropriate licenses).
(1). You can't engage in certain trades without special license or registration.That includes, for comparative example, the medical industry. You can't just get few of your buddies together, write a contract and lay on table to operate each other in the basement of your house. No matter the extent of involvement or mutual agreement to conduct the surgery, each and every one of the participants is guilty of the unlawful practice of medicine. Same with securities trading, no matter how you put it, if he was involved in buying and selling the stocks with the funds of other individuals for profit he is automatically guilty of that deed. There is no way around it, no matter how many pages of paper you waste to write a contract on it.
(2). There are two types of trades, regulated and unregulated.
A. If the trade is unregulated there is no restriction to engage and no mandate to have a written contract to loan the money and offer shares. In fact, if you have a good , trusting relationship a very close person may take it as an insult if you insist on putting it on paper. Of course, for all practical purposes, to avoid unwanted losses and considering we live in XXI century and not in the age of gentlemen of honor, it is most advisable and prudent to put everything in writing, but it is not something you must do or face jail.
B. If the trade is regulated and you lack the license to perform then , regardless of anything else, you simply CAN NOT engage in it and if you do then you must be ready to face the consequences.