3-gang box on separate circuit breakers

Hi,

I have a 3-gang box with 3 switches in it. I just put in single-pole WEMO smart switches to replace the old school rocker switches. They were both on circuit breaker #10. They went as smooth as can be. The 3rd switch is part of a 3-way circuit. It is on another circuit breaker. I purchased a GE 3-way capable smart dimmer switch, and pulled the switch out live so I could get a picture and map out which wires are live and which aren't. I was confused how they were doing a three-way with only two wires until I flipped the other switch and found that the white wire became live, so that is no neutral. This must be the traveler. My first thought was to use the neutral in the box from the other switches, but I checked and this seems to be a NoNo. I haven't pulled the other box apart yet. It's also a 3-gang box, so I'm hoping it has at least one shared neutral wire so I can put a smart switch there. Now that I've said all of that, I have a couple questions:

1. If I replace the existing switch that has no neutral with a more modern dumb decora 3-way switch, is there any safety concerns? That is more so than the fact it already doesn't have a neutral wire?
2. Assuming I find a neutral on from the same circuit breaker for one of the other switches on the box I haven't looked at yet. Is it okay just to split off a neutral?
3. Can I just eliminate the 3-way by capping off or capping together some of the wires?

I use a yellow non-contact tester and the black and red show up live regardless of the switch position and the white wire shows up live depending on the other switch's position. I'm assuming this is common knowledge, but just in case it isn't, the smart switches require a neutral wire.

Thanks,
Jeff