Hey Noel! Here's what I think:

Cross-posting is totally okay in my book. Some people read on Steemit. Some people read on Medium. Others may subscribe to your RSS feed. Think of it like how if you released an album, you'd want it on Spotify and Apple Music and CD, because people have their preferences. Yes, it's inconvenient to keep up with comments, but it all depends what your goals are. Despite the fact that I'm responding to this comment, I personally don't stress about getting to every single one.

I've recently even been RE-publishing old stuff on Medium. This is common practice by people like James Altucher and Benjamin Hardy. Sometimes I even re-publish old stuff on Medium under their partner program, and I earn money that way. It's great, since I didn't earn money on the original post. I haven't found anything in their partnership agreement that prohibits this practice.

I haven't re-published on Steemit, but now that I think of it in theory it would be okay. Both Medium and Steemit have voting systems to democratize the success of good content. If people have seen it before (and don't appreciate the reminder), that would be reflected in the success of the posts.

As you saw, you can still edit content. I don't know much about the technical aspects of blockchains, but I believe that your original post is on there, as well as a record of what was edited. So, that's the part that's immutable.