To get your email threading back to an approximation of the old iOS 9 version:
1. Go to "Settings" and "Mail" and then scroll down to the "Threading" section and select "Most Recent Message on Top"
2. To scroll to a particular message within a thread, use the blue "<" and ">" arrows at the top right of the screen. The "<" arrow goes to older (prior) emails; the ">" arrow advances to newer (more recent) emails.
3. To reply to a particular email within a thread, go to that email, and then swipe left. You will then have the choice of replying to the email (blue circle with reply arrow symbol), flagging the email (orange circle with flag symbol), or deleting the email (red circle with trash can symbol)
Hope this helps! It took me a lot of research to figure all of this out.