Firstly, under an option C contract, the Employer is paying for your Defined Costs plus Fee, so even if there is no adjustment to the target Prices, the cost of doing this is split by the pain / gain. You reviewing has got to be more time consuming than the designers identifying the changes they are making.
I know reverting back to "the spirit of mutual trust and co-operation" is seen as a back stop for a 'claim' / compensation event, but most commentaries talk about it meaning having 'due regard' for the other parties business i.e. not putting their interests before yours, but taking account of the effect of your actions on their business and doing what you can to minimise them without incurring substantial costs or significantly inconvenience yourself. This seems to fall within that category.
How about notifying the PM as an early warning of the above situation as it is costing both you Defined Cost and the Employer monies under the pain / gain mechanism. You might also point out that, as an agent of the Employer, the PM - and hence the Employer - is not acting in a spirit of mutual trust and co-operation which is a breach of contract and a compensation event under clause 60.1 (18). That would mean all cost falls on them. Make it clear, you don't want it to be, you just want the changes highlighted.