Is it permissible to wipe over a head covering during ablution due to an illness that prevents direct wiping on the head, and what is the ruling on the ablution if the head covering is removed after wiping over it?

It is permissible to wipe over the turban during ablution, but it is not permissible to wipe over a skullcap, a cap, or a hat. If there is a splint or a bandage on your head, it is permissible to wipe over it. If the head is exposed and would be harmed by wiping, then the rest of the ablution parts should be washed, and tayammum should be performed in place of wiping the head. Sequence is a condition for minor ritual purity, so the tayammum should be performed at the point where the head would be wiped.