Split-screen video sync issue

Hello! This is a bit of a weird one and will take a bit of explanation - apologies in advance if this is long. (I've looked back through the forum and can't see any similar questions.)

Here's the thing: I thought I'd got the hang of creating split-screen videos in iMovie. It's a bit fiddly, so I presume the software isn't really designed to do this, but it works, so I don't mind fiddling if it means I don't have to buy any new software. Or at least it DID work, until today.

Just to be clear, here is the method I use. (I found a few YouTube tutorials on this, but none of them quite explained everything I needed, so I had to do a bit of experimentation before I found a method that works for me. These notes are mainly so I can remember what to do if my memory is failing me.)

Split-screen video in iMovie - instructions

1. Start a project (create movie) & import all the videos (if they're the wrong shape, crop in iMovie Library, not in project, before using)

2. Go to backgrounds (this will be the bit between the frames) - can be a picture

3. Select one (e.g. stars) and add to area at bottom of screen

4. Go to "info" tab & adjust time to length of video

5. Add title if needed

6. Go back to "my media" & drag 1st video to work area

7. Go to video overlay & change "cutaway" to "picture in picture" to resize & place video (NB leaving gaps for subsequent frames) - can use crop etc if desired - set dissolve to 0 - click OK

8. Tweak sound if needed (or ignore if audio has been mixed separately)

9. Save to file (file -> share -> file) - call it frame 1 or whatever

10. DELETE EVERYTHING FROM PROJECT (i.e. work area at bottom) so it's blank

11. Import last export into iMovie from folder where you saved it - this means the subframe version is now available in the project

12. Repeat from step 6 for 2nd video & subsequent ones - NB each time you will have one extra frame filled in before you add a new one - when you add the new one, do "picture in picture" again to rearrange, then get the two lines in sync & balanced before saving

13. If audio mixed separately, wait till all video frames in place, then delete audio & add mixed audio (NB make sure video in work area has a bit of space at the start, because if imported audio starts before video this is awkward to fix, but vice versa is easy)

Now, here's the issue. Mainly I use this method for multi-part a cappella recordings with me singing all the parts (e.g. this recent one https://www.youtube.com/watch?v=730nacSRo_w ) and it has worked fine. But today I had to put together a similar video for my community choir, which consists of several inexperienced singers all singing the same tune. I didn't expect it to take very long, but after spending seven hours trying again and again, I am tearing my hair out, because the videos won't stay synced to the audio. (The audio in this case has been mixed separately, but I'm using the backing track to sync the videos to as I add them, before replacing that with the mixed audio at the end.)

To be clear: I have 9 videos, and each time I add one of them it syncs perfectly to the audio of the track that's already there. But when I re-import the video with the new frame, the video seems to be no longer in sync. It's only a slight difference, but if I ignore it (as I did the first time I tried, because I didn't notice until I'd added all 9) by the time I've added all the frames they're all noticeably out of sync, and the worst are the ones I added earliest.

Here is the video in question (please don't comment on the actual singing - these ladies are not very confident!) https://www.youtube.com/watch?v=cF_xhRdQWPA

Each of the nine videos was perfectly in sync with the audio when it was added, but seems to have got out of sync during the export/import process. This is driving me UP THE WALL, particularly as these ladies are so keen to see the result and I told them it'd be straightforward to do! I've shown them this draft version and they already like it, but I know it can be better.

(The weirdest part is that I've used this method lots of times and never encountered this issue before, but I'm actually wondering whether it might be possible that it HAS happened before, but I didn't notice because all my previous videos had just me doing all the bits, and I always do the lead vocal last, so if the backing chords or the bass line became out of sync, I might not have noticed as long as the lead was OK. But I'm doubting myself about everything now!)

If you read (and understood) all that, thank you, I appreciate it! But I'll appreciate it even more if you can suggest a solution.... anyone? Please?

(MacOS 10.15.6, iMovie 10.1.14)