Untangling iCloud Mess
This is a common engagement that I see and there is really no way to untangle purchases going forward.

The main thing to focus on in my experience is if you can edit the AppleID or not. If the account in question is @icloud.com, @me.com or @mac.com - it's set in stone and you can't really repurpose it.

If the AppleID is not using an Apple extension, then you have much more flexibility to rename the actual AppleID and update all the security, recovery and primary emails and convert a "personal" account to a "role/work" account and the other way.

When looking at the value of purchases, also consider the billable rate and the time spend not doing other work. Often it's simpler to write off the loss of past purchases and just make a clean break. Putting humpty dumpty back together again isn't viable in most cases.

Specifically:


retaining purchases - no go since they stay with the account and you can't use one Apple ID for that many devices without Apple eventually shutting it down.
fixing iMessage - this is doable. It is a lot of work, but you can plan that work on a piece of paper and then decide if it's really worth it to try and reuse one Apple ID or if you will just make a new account and use that going forward with a complete wipe of all the "wrongly configured" devices.


Again, get an estimate of how much your time is worth and how much time the people using the devices is work and solve your particular situation for the lowest $$ invested.