If there is a mod that directly conflicts with my mod, but the author of said mod has abandoned their project and has either not read or ignored several attempts at communicating with them to come up with a common solution, is it unethical or even against the "rules" for me to edit their mod to create a patch that fixes the problem? My gut tells me it's a navmesh issue and I don't even believe the navmesh edits in the conflicting cell are needed in their mod and should be a very easy fix. I've done everything to my mod I can do to provide compatibility except move my entire prison, and I'm not doing that. I would have to add the edited esp to my optional files for users of his mod to download, but wouldn't include any of the assets it needed.

As I said, I've tried numerous times to contact them to no avail. I would probably also send Robin a note explaining the situation and get his permission if possible too.

I'd suggest making an optional compatibility patch if possible, uploaded alongside your mod, and explicitly state the other modder is simply not responding, but if there are any issues you are free to be contacted. Seems to be what many people do.

Does the ReadMe file say anything about permission of usage?

But when in doubt, just make a patch and include it with your mod.

It's not like the mod is ten years old and the author has announced leaving the community with no plans to return. And an abandoned mod doesn't mean it's up for grabs to be used/modded.