There are basically two kinds of conduct that you identify.
One is backing away from what you believe were oral promises made by the employer and lawyer regarding payment. Whatever the status of the promises made by the employer, the oral statements made by the lawyer would probably be viewed by a court or ethics board as settlement offers or proposals rather than actual binding agreements, and this is unlikely to be considered an ethical lapse.
For purposes of ethics questions and fraud lawsuits, lies about what kind of deal you are willing to make with an adversary don't count as lies. This isn't a terribly logical rule, but is is a well established one.
I was told (by employer), verbally and in text/email messages that I
"would be paid when the deals closed."
It is going to be very hard for the employer to take back those written statements and text and email messages are usually given the effect of signed writings in a court of law. This is going to be taken as a confession of the employer regarding the probably unwritten agreement of the parties regarding your right to be paid on these deals, so you would be well advised to stick to your guns on this issue. The percentages will be another point that is hard for the employer to fight if there is a course of dealings between the parties in which you receive a consistent percentage or there was a written agreement concerning your commission percentage.
Also, even if the lawyer did make a promise and breached it, this would still only be a breach by the employer of a contract made on the employer's behalf by his lawyer. It is not an ethical lapse to breach a contract about future conduct, and a lawyer is not personally responsible for contracts he makes as a disclosed agent of your former employer.
The second is making a false statement of fact about whom the lawyer has discussed the matter with. Lawyers do have an ethical duty to be truthful and failing to do so is an ethical lapse. But, this duty is generally interpreted to apply only to statements of fact which are material. If a lawyer lies to you about how old he is, or whether he's ever had an affair, in the context of a pre-litigation negotiation like this one, the ethical officials won't care. If a lawyer lies to you about something material to the transaction (e.g. claiming that the employer has money in the bank to pay a settlement when in fact it is overdrawn on all of its accounts and has no money coming in and the lawyer knows those facts), this is a serious ethical breach.
It is hard to see how this information would be material, even though it casts doubt on his credibility. Ethically, he owes any duty of confidentiality to his client and not to you, so it isn't your complaint to make from a confidentiality point of view.
Also, unless he discusses confidential advice that he provided to his client when no one else was present to you, he has not waived the attorney-client privilege, contrary to the answer by @IñakiViggers on that issue.
Of course, proving that the lawyer said anything in an oral conversation at which no one else was present comes down to a credibility fight between your sworn statement and his if the lawyer testifies inaccurately about the discussion. A sworn statement from you is proof and would meet the "burden of production" to provide proof in support of your case at a trial, but wouldn't necessarily prevail easily at trial since the judge might not be convinced regarding who is accurate in their account of the discussion (I have avoided the word "lying" because there are a variety of reasons that people inaccurately recall discussions).
What would be the sensible way to use this information to my advantage
while trying to resolve these matters with having to bring suit and go
to court? Is there anything that this lawyer should fear, if his
unethical behavior was brought to light, either in court or to a bar
The conduct you describe on the part of the lawyer will provide you with little or no leverage in your negotiations and is likely to not even be considered admissible evidence in court since it may be considered a form of settlement negotiations.
Your strongest leverage will be the written statements from the employer.
But, depending upon the amount in dispute, it may still make sense to compromise given the time and expense and uncertainty of going to court. Even in the clearest case, you probably only have a 90% chance of winning a contested case, and you wouldn't cross the street if you knew you had a 10% chance of being hit by a car as you crossed, even if you knew that the collision wouldn't be fatal.
Unless your state has a wage claims act that covers you, you may have little or not prospect of an attorneys' fee award if you prevail, and representing yourself when the employer has a lawyer will always put you at a disadvantage in a court setting.
If the amount in dispute is great (e.g. $50,000+), hiring a lawyer is probably worth it. If the amount in dispute is small (e.g. $5,000) you may want to file a suit in small claims court and only hire a lawyer for a couple hours of pre-hearing coaching.