Trimming the parent message makes good sense.

For myself, I find the redirect at the top to be obvious already. It catches the eye straight away and leads into the new topic.

The ability to split multiple posts would indeed be useful. As it is, if you’re late to the party then the damage is done already.
An alternative would be to close the thread upon splitting. It can still be opened easily but requires a conscious effort to do so. Yes, it interrupts the free flow, but then that’s probably a good thing to stop the flurry of posts.

(edit to add) Splitting multiple posts could be done if they can be appended to an existing topic. That would make it easier programming wise (I believe) and also allow for a thorough clean up. If it was done like that, it would be essential that each parent was reduced to a brief outline only, otherwise it reduces the effectiveness of the split (reducing clutter)