Retrospective application of ‘inside IR35’ by client after contract ended

[Protagoras]

My next concern is if they submit - even without my approval - my previously filled tax return is invalided, the years income increased and presumably I’ll be landed with an income tax bill as it will push me in to the next bracket!

It would be interesting to understand what information would be submitted on a P60 in such a case.
For example, would the 'in this employment' pay box have the gross fees paid for the period, and then the tax and NI boxes completed with the corresponding payments?

If the Ch10 'deemed employer' is now paying the employee taxes, does that mean that OP could get a tax refund on PAYE income from OP's LtdCo on the basis that this money would have been taxed at source?