There are two separate issues here, the legal letter charge and the Super, can you document the Super's activities, kickbacks, and breach of Coop rules/job description or other legalities? Can you establish, have you maintained, a paper trail?

The paper trail is the bottom line core of any action you might want to take on any topic that involves any kind of breach, whether it be shareholder, Board, or Management company. Or for that matter Accountants and legal. Anything. That's the first thing any Lawyer you hire will ask for: logs, documents, emails etc. do everything in writing, not by telcall. Keep logs. Of course your Lawyer will also tell you that you should not discuss anything by email and only by telcall. This is not always possible, but important to understand. It is because all your emails, except that which is explicitly noted as privileged attorney client communications (I do not know how they sort this out, legalistically) is 'discoverable' during the pre-trial/pre-deposition discovery process.
If there is concrete evidence then, if you can muster the authority, fire the Super. period. flat out. no negotiations. The Super is an 'at will' employee. You do not have to 'show cause' unless there is some Union contract/protocol your Building has signed. In fact, if they are not Union, you should not under any conditions provide them with any reason for the firing as that will give them a hook to try and sue on. So, odd as it seems, you say nothing to them, or the Shareholders who ask. Just do it and have them vacate. It seems harsh but, you are doing it because they are corrupt, heavy handed, and providing substandard services, so there is no room for a misplaced sense of compassion here from someone who has been screwing your Coop, is there?

Billing the Shareholder for legal.

First off needs to be stated in your building docs - Bylaws, house rules, etc. that any cause of action precipitated by a shareholder action is billable against the shareholder if, by their actions or activities it has caused the Board to utilize legal services for advisement or to adjudicate the situation. This could be anything from advice, to letters, to actions, court costs, collections etc. it also applies to having to hire any professional outside service such as Engineers, Architects, CPA's, other Lawyers, Litigation Specialists etc. However, it presumes that the Board is operating legitimately within the standard business rule, and reasonably as well. Your citing of the incident makes one speculate as to the legitimacy of the charge-back. That could be due to your citation representation or objective facts. You must always be careful that your personal assessment and feelings which can be subjective do not supplant a true 'arms-distance' objectivity as to the facts. Everything should read the same from both sides of the table. The Lawyer doesn't have the right to charge it back directly, only the Board does. Is your Board dysfunctional? Can you muster a shadow cabinet to run against them at the next election? Can you collect proxies to help? You do have a right to the contact info of your neighbor shareholders from Management, but it might be wiser to go door to door and collect whatever info you need that way. Most shareholders not in residence tend to give a favorite neighbor their keys and contact info. It's all very well to say these things, but documentation is reality, and the basis for any cause of action, and not so easy to come by without some hard and careful work over time. Good luck!