Oomis,
For the most part, whether or not something is “erva” depends on the customs of the community. Any body part that is normally covered is erva if uncovered, therefore scalp is certainly erva in our communities. According the R’Moshe pshat in “saar b’isha erva” is that the Gemara is teaching a chidush that hair could have the status of erva even though it isn’t skin.
There is a separate chiyuv for a married woman to cover her hair (not because of erva) learnt out from the posuk by Sotah stating that the Cohanim uncover the head of the Sotah. The posuk uses the the term “rosh” not “saar” so I can hear an argument that it should apply to a bald head as well.