How can the excommunication of a schismatic group be lifted, for the purposes of reconciliation? If no reconciliation happens, does the excommunication resume?
Excommunication and Schism refer to two distinct things.
Excommunication is a punishment leveled for various offenses; sometimes it is latae sententiae (of the sentence being passed; that is, automatic as declared in the law) and other times it is ferendae sententiae (of the sentence to be passed; that is, must be declared by some judicial process).
Excommunications can be given for various acts including schism.
Schism is a refusal to assent to the authority of the Supreme Pontiff.
Because an excommunication is a punishment, it can be lifted by the authority who declared it or their superior in law. In the case of automatic penalties, the law defines who has authority -- some such penalties are reserved to the Apostolic See.
A punishment could be lifted as an act of mercy, similar to a granting of parole or even a pardon in civil law. This act of mercy could be done as a way to bring about reconciliation in a different way. The purpose for a penalty like excommunication is to warn someone they have gone too far and call them back to communion.
If no reconciliation occurs, there is no automatic way for the excommunication to be reinstated. It would either need to be directly declared or that party would have to commit another offense.
Remission of excommunication is discussed in
1983 Can. 1358 §1. Remission of a censure cannot be granted unless the offender has withdrawn from contumacy according to the norm of can. 1347, §2; it cannot be denied, however, to a person who withdraws from contumacy.
Thus, a schismatic group like the schismatic Eastern Orthodox would have to withdraw from its contumacy of refusing submission to the Roman Pontiff before its excommunications can be remitted.
One is said to have withdrawn from contumacy when one truly repents of the delict committed and at the same time gives appropriate satisfaction for the damages or scandal or at least sincerely promises this; the judgment about whether or not the penitence is true, and satisfaction is sufficient, or the promise concerning these is sincere belongs to him from whom absolution of the censure is requested.