I think it is worth learning Elm first because it many ways it is much simpler. No JS "this" issues to worry about. No props vs. state. It is very opinionated. But it is also virtual dom and "component based" so the paradigm is instilled in the brain to go across and learn React.

Disclosure: I learned some Haskell before Elm so I may be "immunized" against some of the struggle!

I don’t know, I think it’s better to just dive straight into what you’re trying to learn, otherwise you’re basically procrastinating.

I think it depends. For a hard deadline, yes.

For building understanding and mental models, I think it can be bad to jump into things that are too much of a struggle.

However the best of both worlds is some kind of tutorial that explains it really well. Like the recent "explaining functional programming to my 6 year old" article. Then you can dive straight in, but get off to a solid start.