How to make most of incompatible mods compatible with reversenx-rt (including mario odyssey splitscreen)

Copy pasting this from a recent comment on reddit for a post related to another overlay :

SaltyNX is incompatible with certain mods that do exefs patching; specifically the file SD/atmosphere/contents/[title id]/exefs/main.npdm needs to be compiled with allow_debug=true, force_debug=false, force_debug_prod=false. In my case, I downloaded the source for the mod I was using and (with some minor help from its devs) was able to make the necessary changes and obtain a new main.npdm file that was compatible with SaltyNX. It looks like it doesn't break anything with the mod, and the devs will look into making that change on their end of things once they confirm it's all good.

To check if this is the issue you have with SaltyNX, here's what I recommend:

Try a different game without any mods. Make sure it's supported by SaltyNX (e.g. Mario Kart 8 Deluxe is not supported by SaltyNX). If it doesn't run on an unmodded game which should be supported, it's possible you have issues with your installation or configuration with other CFW. Delete the SD/SaltyNX/ folder and all of the relevant .nro and .ovl files from the SD/switch/ and SD/switch/.overlays/ folders, and reinstall SaltyNX, FPSLocker, ReverseNX-RT, and ReverseNX-RT Tool via the Homebrew App Store.

Try disabling any mods. Some simpler game mods install just a few files into SD/atmosphere/contents/[title id]/, or elsehwere in SD/atmosphere/, and can be managed with the SimpleModManager tool. Here, the [title id] represents the unique 16-character ID for each game, there's lots of ways to find it out yourself or just by google. I never modded TOTK or BOTW, but I think they require external mod loaders to set things up. Follow whatever guides they provide to make sure you don't leave any of these files behind that could muddy things up. If it still doesn't work, there's probably something else going on, but I couldn't tell ya what.

If you're using several mods together, try enabling ONLY ONE at a time and test the game with only a single mod enabled. This can be tricky or time-intensive with TOTK/BOTW needing to rebuild mod files depending on which mods are enabled and such. If it is these debug flags causing issues, I would think it would be introduced by a specific mod rather than the mod loader itself, or this would likely be a wider known issue.

Once you're able to find a mod that stops SaltyNX from running, look in SD/atmosphere/contents/[title id]/exefs/. If there's no main.npdm then I guess it's something else. But if there is, you can check with a hex editor (e.g. https://hexed.it/) and look at the very end of the file. If you see something other than FF FF 02 00 then the debug flags aren't compatible with SaltyNX. Congrats! You found the issue!

It's worth noting that, for some mods, it might work to just edit the hex data directly so it reads FF FF 02 00 at the end, originally from MasaGratoR's advice. In my case, that change at the end of the file just caused the game to crash when loading ("Software was closed because an error occurred", not an Atmosphere crash). After modifying the mod files source code and rebuilding them, I found that this string FF FF 02 00 ends up changing twice, once at the end, but also somewhere else earlier in the file.

Source : https://www.reddit.com/r/SwitchPirates/s/VXcOrpQ9oV

As an example, mario odyssey multiplayer main mod file ends by FF 04 00, replace this by FF 02 00 and search in the file for the other FF 04 00 then replace it too

When asking with wrong words if there could be a fix (I've asked if it could be fixed, the overlay, not knowing that the mod had to be edited not the overlay) Masagrator said to me "Why should I fix compatibility with some mod?" and straight out closed then deleted the issue from github (it's nowhere to be found anymore except in mails)

Ironically check out who gave this fix to the reddit user...

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