what is the chiddush that he can do chalitzah, why would we think that he could not do chalitzah?
In most cases where yibum is forbidden, chalitza is not done either. For example, if a man’s daughter falls to him in yibum, no yibum or chalitza is done. This is because she is an erva to him.
It is possible, however, for a KG to marry a widow. In other words, if he does it, it’s a valid (although forbidden) marriage. Since a marriage *is* possible, chalitzah must be done.
Interestingly enough, WRT a kohen gadol, he could even, m’ikar hadin, do yibum, since the first act is an aseh which overrides the prohibition. However, since any subsequent conjugal acts would be forbidden, we do not allow him to perform the first.
The Wolf