I am afraid it is back to the beginning with flash photography.
I have posted 2 images with some dreary Santa and using flash.
The conditions were poor to say the least but no excuses.
I feel certain that the faults you see are the same as the faults I see. Would you please cast your eye over these abysmal images (the whole shoot was of the same low quality. The previous topics you corrected need not be repeated.
My main concern is how to get my usage of flash up to scratch with guidance on eliminating the colour cast. My flash unit is a budget level unit.
Flash sync mode was front curtain with an EV of -1.5 which was necessary to eliminate overblown highlights and WB set to flash. Santa and the decor were in warm colours but came out too warm.
These shots were captured in RAW and only converted for posting to my gallery. I noticed that even without post processing artefacts are present.
Despite your earlier clarification I notice the camera has drifted to AF area mode closest subject. As far as I am aware this came up when I tested the conditions with an Auto setting as a start point and this remained when I switched to Shutter priority (I should be shot for not checking).
The topics previously covered by you, in depth, need not be repeated but any pointers concerning flash correction that you indicate will be most welcome.