Your description is very confusing, hence the request for pictures.

Sorry. Each old switch currently has 3 wires connected to it, only one of which is labeled (common on both switches.) The switch with a bundle of neutrals has a red wire connected to the common, another red wire, and a white wire (the only hot one.)

The switch without a bundle of neutrals has a black wire connected to the common (hot), a white wire (hot), and a red wire (not hot.)

On the side with the neutral lines you need to use a multimeter to test voltage between neutral and the 3 conductors on the existing switch, one will be hot. That is your line

I use a Fluke voltage tester to see which wire is hot with the light off, would this also work or should I get a multimeter?

To just use the single switch as the main: Connect all three wires from the slave switch together. Install the new switch with the neutral, line connected to incoming line and load pigtailed to the two travelers from the previous 3 way switch.

Just to verify; in the switch without a bundle of neutrals I would connect all 3 wire together with a wire nut, and then in the switch with a bundle of neutrals I would connect the neutral to the new switch, connect the line wire to the line terminal, then wire nut the two travelers together and pigtail them into the load terminal? This would make this one switch activate the light alone?