As long as you don't use Gateway with your sysNAND after installing B9S, then there's no risk. There's no reason to be worried.
You should
backup your Gateway save files before you do anything else. Then you can install B9S and Luma3DS like normal.
Could you explain how it's better?
Yes, we're talking about how much space games take up. .3ds games generally take up more space.
I like the Gateway cheat engine, but it's functionally the same as other cheat engines. Have you used other cheat options? What you're saying sounds like something people used to say back when Gateway had the only viable cheat engine, but that's not the case anymore. Also, if OP doesn't care about the cheat engine, then this is irrelevant anyway.
Yes, because Luma3DS is compatible with all 3DS software versions. It's seems pretty convoluted to setup a system in which you install the games you want to run on 11.5 to the 3DS SD card for use with Luma3DS, and you install the games you are okay with running on 11.2 onto the Gateway SD card.
Ignoring that the last 1GB on an SD card can be a big deal to a lot of people, using an emuNAND reduces the lifespan of the SD card. SD cards aren't as resiliant as NAND chips when it comes to reads/writes.
That's the part that's risky. If you accidentally boot into Gateway mode using the B9S sysNAND instead of the emuNAND, then you've just removed B9S on an 11.5 sysNAND. That means you'll be left with no entrypoint with which to boot Gateway or Luma3DS. You'll have to find one of the 11.5 methods of installing B9S (ntrboot, DSiWare method, or a hardmod) as if from scratch.
Don't hold your breath. Soon™