You might first add a Research Note section to each of the affected profiles, giving your sources and reasoning for the proposed changes. Then you could contact the PMs and refer them to the profile notes, and if this is something you're willing to tackle, offer to make the changes for them.
These are pretty major changes so i wouldn't just do it without giving the PMs a shout first, and a reasonable time period to respond. If you don't hear back in say a week or two, go ahead and make the changes.
What you call a "manual merge" is really just moving data around to the correct profiles. You'll end up with the same names but with all their data in the right place and connected to the right parents and children. I see nothing wrong with this approach, and it avoids an unnecessary redirect.