As expensive as it may appear, I would suggest that you start from baseline and replace the entire IU set, along with the toners. It would be beneficial to ensure that the transfer belt and second transfer rollers are also new, or at least have not been reset for life count. Sometimes an imbalance in any IU can really throw things out of whack as the machine tries to compensate.
Be sure to clean the IDC sensors.
And reset ALL colour adjustments in tech rep. Everything back to zero. Factory reset cannot hurt as well. In fact, do this before any new consumables have been put in.
I think firmware was mentioned as a must do.
Reset and stabilize should generally level the playing field for each IU. If third party toner is suspected, doing a toner add will not flush the toner, it will merely add (top up) the toner level to at least 7% or so. Adjusting the toner level setting will increase this value. Have you checked that? It is always possible this could be the problem. Toner density adjustments can also throw things out.
As for developer mixing into the final print, it is possible. But it would also be obvious as the printed colour half tone would look gritty and feel gritty on the page.
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