I have constructed berms and modified slopes in my garden, and I have not had any problem using 1:3 (rise/run) slopes or 1:2 slopes. The key to preventing erosion is to ensure that you have sheet flow down the slope. If the face of the slope allows for concentration of water in a valley or pooling, then the chance for erosion will increase. Depending on the area drained and concentrated the amount of eroison may not be significant. I have a different issue, with some of my berms, other than erosion of the slope by water flowing over it. The base of my slopes after a few years have some minor blow out. If you buidling your bed with material similar to the underlying ground, then you probably won't face any noticeable problem.

You can either depress the bathroom subfloor slightly as you mention or build up the vinyl floor areas with plywood or something. You could also float out at the doorways but ideally you should keep the slope at less than 1/8" in 10'.

I'm not fond of a wood transition between two tile floors. Of course it is an easy install...ahem...easy on the installer that is. The Schluter system is correct. It has been used hundreds of thousands of times (should I say millions of times...who's to say the number but it is A LOT!). If a slab that has concerns with cracking/movement (and they all have SOME sort of movement whether it be a little or a lot....so be aware) should have a decoupling membrane. And just to be clear carpet HIDES some HORRIBLE substrates. I mean *GASP* SPUTTER* HORROR* type concrete. Why you ask? Because BUILDERS know that carpet hides HORRIBLE workmanship. Ergo a carpeted slab will be MUCH uglier than a slab prepared for tile. Here's what you are looking for in a tiling professional: https://www.tile-assn.com/ You want to WRITE OUT the contract so that it states: "To be installed per NTCA Specifications AND per tile manufacturer's instructions." And a 1 year labour warranty is standard stuff in the building industry. You are looking at a substantial installation (if we estimate the labour alone will be $10-$12/sf and the tile/materials will be EXTRA) somewhere in the range of 500sf - 600sf (or so)??? Your quote MAY NOT include "subfloor preparation". If it DOES....please ask for the guestimate they have provided for the subfloor. If they have estimated $0.50/sf for concrete work, then they are too low. If they have estimated a MAXIMUM of $4/sf (not to exceed this amount without your written approval) then they are more in keeping with reality. I would stick with the person who is well acquainted with the NTCA guidelines, if not a member, the person who INCLUDES the cost of a decoupling membrane, who INCLUDES the high-priced estimate for slab prep as part and parcel of his quote....and then the person who includes the Schluter system as part of the transition between floors. Please notice how the transition is FAR down the list. Right. There are some more important things that need to be dealt with other than the final look. And Schluter has DOZENS of transition options which work with floors AND/OR wall settings. The appropriate option will be looked at before anything is laid.

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.