Ethical rules for lawyers are enforced by a separate committee or board or department (states vary) to which complaints can be made by lawyers and members of the public.
If a complaint appears to have merit, the board has someone prosecute the case in a quasi-criminal manner, which the board hears like a court, subject to an appeal at some point. The consequences range from an order to attend an ethics course, to publicly or privately scolding the lawyer, to suspending the lawyer's license, to disbarring a lawyer, depending upon the severity of the offense.
The board interprets the rule in course of the deciding the case.
This doesn't provide any relief to the aggrieved party. It merely protects the general public from bad lawyers.
There are many interpretive principles and precedents for interpreting what violates Rule of Professional Conduct 4.1.
if a client makes false written statements about key dates (say a week
long period) in matters of fact and the lawyer knows they are false,
what are their ethical responsibilities?
If statements are made to the Court, Rule 3.3 (candor to the tribunal) applies, not Rule 4.1.
Another example, in matters of divorce and custody, is a lawyer
expected to make truthful statements in court regarding the nature of
custodial interference?
It would be unusual for an attorney to make a statement of fact to a court on any issue. Usually that is reserved for witnesses.
