there is some misunderstanding about this process, if the league wants to unilaterally impose a cap, they have to do this in steps. First of all, they have to try to negotiate a cap with the union {done}, they have to get to a point where it becomes obvious that no further negotiations would ever resolve the situation {just about done i'd say}. At this point, that would declare an IMPASSE, and their LAST offer to the union would become the NEW CBA, it's not that they would imposse a cap without a cba, they would, instead, have to have a cap proposal in their final offer to the union, then declare an impasse saying that negotiations have broken down permenantly, and their last offer becomes the defacto CBA.
Once that is done, they can impose the cap on the players. The union, can then go before the tribunal and declare that the league has not negotiated in good faith. the new CBA imposed by the union is in effect until the tribunal rules on this. If the tribunal rules in favor of the plaers, then we go BACK to the old cba, and no cap again {this is where it starts to get very ugly}. If the panel rules in favor of the league, the union can then declare a Strike. So now, the league would have a CBA, with a cap, and the league would be on strike.
Once all that happens, the league can, once the union has gone on strike, bring in replacement players {they can't do that before the union goes on strike}. And then it becomes a showdown, will the fans pay to see scabs play, or will the union members walk through the picket lines and resume play. Either way, at that point it will get resolved pretty quickly, it's the mess in between we have to get through, unfortunately.