It sounds as if you Mother did not fully understand the forms when she applied 22 years ago and now wants to regularise her position. At best she made a mistake.
All she needs to do is contact her social welfare office and explain her situation. And fill out the current form correctly. She has taken advice from her accountant, her solicitor, family and now the internet, so is aware her current payments may be inaccurate.
To accurately assess her non contributory state pension she needs to declare all assets belonging to herself and your father as they are a
couple. Her accountant sounds as if he recommends she progress as a single person which she clearly is not. Her solicitor has said declare all joint savings and this is what she should do.
It sounds like she retired so maybe she had contributions to a contributory pension and would have been entitled to that which social welfare can assess. If she made an error/mistake on one form 22 years ago that lead to 22 years of over-payment then I am sure social welfare will work it out and assess the overpayment and how much they can recover in the future. If however they discover your mother filled out subsequent forms with the same error/mistake then they may take the view she was dileberatly misleading social welfare and could prosecute her.
With three saving accounts your parents are in a good position to repay the state if social welfare deem she owes them money which is good for all us citizens. How they go about that will depend on their
rules and regulations, but you should not be surprised if they decide to ask for a cash settlement of your parents savings -€20K, and statements of recent transactions to ensure money was not moved out in advance.
The other good thing is that the state will cover the cost of nursing homes if your parents cannot afford it themselves.