“Am I correctly understanding from the sources offered here that the Mechaber opposed sheitels but the Rema permitted them? ”
There is NO Mechaber opposing, in fact the Rosh Hamatirim is a Sephardi i.e. the Shiltie Giborim…
And as I quoted there are many Sphardi gedolim that do not agree with the Yabia Omer – see the above Teshuva of the Yaskil Avdi and the Kaf Hachaim as well in CH 71