Short answer is that you can't, ctr-httpwn was patched on 11.4. My firmware was still on 11.2, so I could still use it, but I thought updating to B9S broke it, but I was mistaken, the error message is just a bug. If you're already on 11.5, the only way to use it is to restore Nand backups to 11.3 or under, but I've been told by many that if you already installed A9LH or B9S before you updated to 11.5, then you don't need to worry if Nintendo put out another update since they can't patch those (but update to B9S if you haven't already).
1. I wasn't even talking directly to you about that, but it was a response in regards to you stating someone would tell me there's no point downgrading. My response to someone telling me that is that if I want to downgrade, I will if I want to whether someone else thinks I should or not because I would do so for a reason.
2. Technically, yes it is impossible to downgrade from 11.5 because the whole thing that allowed doing the downgrade process was patched in 11.3, I think, maybe 11.4. I do have backups of my NAND at 11.2, 10.3 (I think), and a hardmod backup of 9.4, I believe, so I could restore an old NAND if I needed to. I keep them in case something goes wrong, or if there's something that I'd like to install but the installation method was patched or something, I don't keep up to date on all this, I just like having the options open whenever my interests might turn to messing around with homebrew.
3. It's impossible for ctr-httpwn to run alongside extended RAM games on the old 3DS, of which Sun and Moon are, yellows8 himself said so. The hax is cleared as soon as the game takes over the extra RAM, just like entering System Settings as it's like resetting the home menu or something. You see this if you try to use a code in the mystery gift menu. As soon as it tries to connect to the internet when you use the get gift with CODE option, you get told to do an update of the system, and also the game if it has one. Getting a mystery gift with the internet option, like with Tapu Koko doesn't trigger that because the code option connects to Nintendo's servers and the normal internet option connects to Gamefreaks' servers.
4. I didn't start this thread for you to reply and start a fight with Lacius by immediately calling him out by name about the downgrade thing, for either of you or him or KunoichiZ, so I asked you guys to just drop it. As a matter of fact, when Lacius responded, he didn't even care or tell me what to do anyway.
I just wanted to get help and figure it out, then post the solution, and I did do the same thing you were suggesting anyway, but not because of that, I always make backups before updating.