It's been a long time, but I think you're correct. In my environment I didn't actually care (any failed push would be retried so the layers would always eventually complete, and anything that for whatever reason didn't retry, well, it didn't happen enough that we cared at the cost of S3 to do anything clever).
I think OCI ordered manifests first to "open the flow", but then close is only when the manifests last entry was completed - which led to this ordered upload problem.
If your uploader knows where the chunks are going to live (OCI is more or less CAS, so it's predictable), it can just put them there in any order as long as it's all readable before something tries to pull it.
At this point you’d validate that the layers exist and have been uploaded, otherwise you’d just bail out?
And those missing chunks would be handled by the normal registry GC, which evicts unreferenced layers?