Wrong. All wrong. The ONLY way they could stop you from distributing your mod is if it contained actual content that they own - like graphical or audio resources.

Many mod for skyrim is made using the skyrim modding tool at the very least for the packaging. Or made using proprietary file format.

Wrong. Valve told him if his mod depended on another mod, that was okay as long as the other mod was downloaded separately.

Valve effectively told him that, but this is sadly wrong. If I create a ressource or tool and forbid anyone else to use it in their mod, then I have the right to prevent you from publishing your mod if your mod use any feature/asset of mine. The fishing mod was NOT okay if the fnis behaviour file was a required file, even if said file is not downloaded in the fishing mod. If the file was only "optional" but add built in compatibility with it, it's up to a court to judge if the animation was part of the core product sold on the workshop or not. See pear PC vs apple for a related case.

It's very simple. Is there content in your mod that you did not personally create? Then you need permission. Did you personally create all the resources in your mod? Then it's 100% yours.

Yes, but you must also take into account the tools and the context of the mod creation. If you created the mod while working at a company because you had nothing better to do at the moment, then the company technically own your mod. If you used the skyrim modding tool in any way, then bethesda also own part of your mod.

Depends entirely on the license. Your blanket statements are WRONG. If the mod license of a previous version said you were free to rehost it or share it, then you can do exactly that, even if it's gone paid.

Well no you are just trolling. Of course if the author gave the permission to rehost it you can rehost it. just read to lines below : "Most proprietary license forbid you from distributing the work of the author. This is entirely up to the author to allow it or not."