Hmm, you saw my reply? As I thought I deleted it. I had a nice long procedure written for what you wanted to do, but was worried about the risk of data loss, which would then make you feel you get bad advice in the Elastic Forum (I don't work for Elastic) and be mad at everyone. So cool as the idea is, I was wary of outlining how I'd do it (quite a few steps and you'd lose your source as soon as you split).
On restore, progress, I don't know. I'd think you'd see the indexes created as it does and datastore size being used, etc. It's a segment restore, so you can see segment count in your source & destination clusters to give you some idea of how far it's gotten (segments can be different sizes, so it's not a true data %, but would given an idea, I'd think).