You can also chamfer the edge of the hardwood. Probably easier than chamfering the marble unless it is a small chamfer. A larger chamfer over a portion of the threshold is common for differences as small as 1/4 inch. The threshold is simply turned into a small ramp to ease the transition and eliminate any edges.

Would you recommend a rubber underlayment? It appears a 1/2" rubber underlayment from the rubber company is the same product as the branded Serenity Mat, etc for a lower cost. To get an idea, I purchased a 6'x4' 3/4" rubber mat which was also a recommended solution. I can see this accomplishing airborne noise but it was quite rigid and created notable vibration. Perhaps the 1/2" will better absorb impact? In the case of a rubber underlayment, is there benefit to adhering the underlayment to the floor with Green Glue or is this a waste of money?

Sometimes, transitions are simply a fact of life. A good contractor can eliminate or reduce some transitions. But depending on existing conditions that's not always an option. You rarely have the luxury of being able to redo the entire level of the house. So, having small steps is just something you might have to accept. And realistically, if done well, it's not going to be a huge problem. Ask your contractor for suggestions how to make the transition look appealing. It's generally less offensive, if the transition coincides with a door way. So, if you have that option, take advantage of it.

You have vaulted areas between the floors that can and will create an echo chamber with hard surfaces in BOTH spaces. Right now the carpet is dealing with as much noise as possible. As soon as you remove the carpet you will notice echos traveling all over the place. And if your vault/space is BIG ENOUGH (yours might be...hint hint) you can get an irritating 'hum' or 'buzz' going on. This can be there even when there is NOTHING creating sound. We hear this in large stadiums, old libraries (with stone flooring), churches, etc. The easiest way to figure out what the carpet is dealing with is to REMOVE the carpet and then 'live' with it for 2 weeks. Have people in the house moving around on the plywood subfloor. All the banging, stomping, echoing will be there with LVP and with tile (and possibly hardwood as well). The space was designed for carpet. The carpet was used in the design to 'knock down' the noise and to prevent it from disturbing the 'up-down' living situation. I would be very cautious about moving away from carpet in the mezzanine/hallway areas.