So whilst this sounds like a disorganised clusterf*ck, based on my understanding the P45 situation and tax situation sound correct. It's perfectly legal to issue a P45 before all redundancy payments are made, and any payments made after the P45 is issued must be taxed on the emergency tax code (0T). The employer MUST NOT issue more than one P45, and the timing the P45 should be issued with depends on when the employment legally ends (which again, may well be before all payments are made).
The employee can reclaim any overpaid tax (due to the 0T code) from HMRC.
I'd suggest the affected people get advice from ACAS.