Feel free to use any method both will work for 11.5. using this method you are actually updating the emunand to 11.4/5(even if not complete), but if your end need is online access, you are free to choose whatever you seem better.
Just before booting luma press select and you can all options there and check whatever you need. also after following that guide follow this https://gbatemp.net/threads/tutorial-using-luma3ds-gateway-on-b9s.431691/
stay away from gateway fastboot. Just use the 3ds guide to install b9s and use gateway and luma side by side (https://gbatemp.net/threads/tutorial-using-luma3ds-gateway-on-b9s.431691/)
It selectively updates emunand so its not same as spoofing. GW can supports native game spoofing which means without updating emunand you can play .3ds games. Why new emunand wont work is because ninty changed few things in 11.3+ updates and GW haven't implemented any workarounds or you may they...
For boop(via network install) you dont need to store cia on sd card. If you want to directly install with fbi on sd card, then you have to copy the files to sd card.
Sorry I don't understand your question? You can use FBI for ex to install cia. If you want to install cia files by network you can use fbi and https://gbatemp.net/threads/release-boop-1-3-0.455219/
On gateway mode or emunand no, but if you can use that same emunand with the cfw and use that dlc. Emunand isnt the issue here that dlc is not supported because gateway has to update their launcher.