The MLBPA would likely assert that MLB violated the March agreement by not in good faith trying to schedule as many games as possible and ask for damages, including lost wages for games not played.
The CBA Appendix B spells out Rules of Procedure for grievances. The following comes from a Fangraphs article…
“Those Rules are pretty lengthy, so here are the pertinent bits: the legal rules of evidence don’t apply, the arbitration panel sets its own standard of proof (in other words, how much evidence one side needs to present to win), and it’s possible to avoid a hearing altogether just by both sides agreeing to submit legal briefs. Also, there are three arbitrators: one selected by the MLBPA, one selected by MLB, and a neutral third party who is usually a lawyer with some experience in conflict resolution and who serves as the panel chair. That means that, as a practical matter, it’s the panel chair who decides these cases.
“It’s also really important to win the arbitration, because suing to overturn the result is really hard. The U.S. Supreme Court held in 2001 in a case called Major League Baseball Assn. v. Garvey that an arbitrator’s decision will be upheld so long as it isn’t the result of fraud, even if it is ‘silly.'”
https://blogs.fangraphs.com/what-the-mlbpas-grievance-means/