In that case I'd suggest testing the merge by uploading Joplin Cloud data to other cloud, say OneDrive and then pointing the outdated iPhone to that new cloud as well. It's going to take a while but I think it's the safest approach in your situation.
If things go well, you can restore to current situation and merge with your live Joplin Cloud. If not, the damage wouldn't be irreversible.
There are a few tools that might help you with eventual conflicts: