I, too, agree that you need to hire an attorney to work with you.
Here are some things you can do now to facilitate the process.
Gather every scrap of written documentation you have that is related to any agreement between your wife and the medical practice. Preferably, the parties to each document and the date of each document are available. Documentation includes letters, agreements, contracts, email, text messages...
If there were oral promises, at least describe to the best of your knowledge who made the promises, when they were made, and who else may have known about them.
Gather copies of your tax returns showing any income received from the business. This includes all supporting documentation and schedules going back as far as available.
Included copies of any information that may have been provided to your wife about the finances and financial condition of the practice.
Prepare a timeline of events from her starting to work with the medical practice up to now. In particular, was the offer of ownership made around the time the COVID loan was in default?
With respect to the promised retirement plan, provide any information that show she was accruing a benefit. In particular, have documentation if the offer of partial ownership was in place of the previously retirement benefits.
Be prepared to respond to the question of why she did not ask about her retirement benefits earlier.
Gather similar documentation about the promised dividends. Note that corporations pay dividends to shareholders. Your wife should have documentation of the number of shares that she owns, and of how she acquired (or was given) those shares.
With this information in hand, schedule an interview with a reputable attorney from a reputable law firm to discuss your wife's case. If you are not comfortable with how the discussion goes, approach another attorney or law firm. You may wish to ask whether the attorney sees this as a likely case of employment law, or a case of fraud on the part of the other owners, or both.
You commented that "if she leaves her job, we lose everything." The attorney can provide some guidance to what extent, if at all, this may be true. Depending on the terms of the agreements, walking away may be the better option. You also need to be prepared in case the medical practice can force your wife out of the business.
Full disclosure - I am not an attorney and this is not legal advice. I am sharing with you the kinds of steps I have seen others take when confronted with seemingly impossible situations.
You have a difficult and stressful road ahead. You will need help to navigate the way forward. Stay focused on the facts, get help, and may you find peace.