A lot of thoughts incoming…
Having thought about the lightsaber some more, it could be Leia who gave Maz the lightsaber. During the Force vision, use the trailer shot of Leia accepting the saber from Maz, but reverse the shot so it appears that Leia is giving it to Maz to get it away from the Resistance.
This implies that Leia knows and trusts Maz as much as Han, but to make this more clear I could delete some lines previously in the film, such as our heroes getting the location of the Resistance Base from BB-8, meaning that Han’s contact with Leia is their only option for getting to the Resistance. Cut out Han’s lines about the First Order finding the Falcon on their scanners, and leave only the line ‘You want to get BB-8 to the Resistance, Maz Kanata is your best bet’. This makes it very clear that they are here not to get a new ship, but to get the location of the Resistance base. After all, why would they take the easily-trackable Falcon to both the Resistance base and Luke’s planet if it’s such a risk?
So how does the First Order find our heroes at Maz’s castle? With no dialogue about scanners or any First Order informant, there is still Kylo’s knowledge that they are traveling with Han, and could preemptively assume where they are headed. But there could be a more unconventional solution in addition to this one. I could place Kylo’s prayer between when Finn leaves the castle and when Rey begins to hear the screaming voices. Kylo is essentially calling out for Vader’s power, and the lightsaber is responding to this evil call, which is why such a thing has never happened with a lightsaber before. I imagine the sequence to be:
Finn leaves the castle
Kylo prays to Vader
Zoom in on the closed chest, with ominous sound effects
Creepy children’s screams make Rey turn around
It is established that items strong in the Force act as homing beacons, as we see with the Dagobah tree in Empire. But Kylo has simply taken this to a new level, and this could be how he has found items such as Vader’s burned mask.
Of course, this all would still work if the lightsaber voices were activated merely by Kylo being near.
In this version, Han’s confusion about the lightsaber could stay, leading the audience to try and work out the specifics with this new information and hopefully being more successful in the attempt.
Finn could still be the one Rey’s going after, even if I kept in her dialogue with BB-8. ‘Leaving’ could mean leaving with Finn. What would make the scene better in any respect is if there could be a way to insert the ship Finn was about to leave on in the background when Rey says ‘You have to go back’. It would be cool to have a matte painting of the front of the ship. The lighting even works in favor of this idea, though it would be tricky but not impossible with the moving camera.