must all interaction be through a lawyer after receiving the first
Consistent with others' answer, no, you don't need a lawyer. But your question in and of itself is indicative of the steep learning curve you would need to undergo in order to avoid "shooting yourself in the foot", as the saying goes.
By this I am not encouraging you to get a lawyer (in fact, here on stackexchange and elsewhere I promote litigation in pro per). Instead, I encourage people to learn about the applicable statutes, procedural laws, how to conduct legal research, and to draft/present their arguments in court.
Here are some suggestions regarding your response letter:
- Avoid sarcastic admissions such as "Right, for sure I am at fault for the employer's [fill_in_the_blanks]".
- If you ask for a clarification, clearly state that you expect reasonably sufficient detail as well as any and all records that substantiate the alleged damages. Although that won't strictly limit the allegations the employer can make in court proceedings, the attorney's reply might help evidencing the employer's vexatious approach later on.
- Avoid wording that may be misinterpreted as consciousness of guilt.
- Be assertive and truthful. Keep in mind the lawyer is gauging (1) how easily he can intimidate you, and (2) whether he can make additional claims to harass you via court proceedings.
- From now on, all your interactions with the attorney and the employer should be in writing (preferably email, given its reproducibility). When unethical individuals are aware that their position is devoid of merit, they are very tempted to indulge in false accusations (of threat, for example). Thus, communications in writing constitute objectively verifiable proof of who is acting unlawfully.
- Even if the attorney premises on your contract (or employment agreement/manual, or company's guidelines) the alleged damages, the clauses at issue might be illegal and therefore void. For instance, from 2007-2012 my former employer (an Indian IT intermediary) prohibited me --via contract-- to disclose my salary. The contract contained the typical lawyered babbling, but that doesn't mean that all of it was legal. In 2013 I realized that the prohibition violated Michigan law, and he had no option but to strike the entire clause. That being said, I didn't sue him for that, but for other more important matters which are currently pending review in the U.S. Supreme Court.
Absent any further context in your inquiry, it is hard to make additional suggestions on how to proceed.