In short, most of what you have narrated is in relation to the practices of Sahabah to what was still inherited from the life around them not just in Arabia but around the world. There is no direct verse or direct authentic and clear Hadith that states the A’wrah of a slave female is different from a free girl.

Verse 33:59 that some used to say it is for the free women only is disputed among scholars. Ibn Taymiyyah says it is only for free women but Ibn Hazem says it is for all females regardless.

In other words, Fiqh scholars had different views depending on what they perceived as a practice accepted as being the status always or something that the ruling can change based on time and environment. Here are my comments in general:

1- Slavery and such actions were practiced all around the world and were not invented by Muslims.

2- Imam Ibn Hazm (check his book: Al-Muhalla,) was against all that understanding and wrote lengthy how a female private areas are standard for all females regardless what status they are in. Also Imam Al-Albani supported this view.

3- This means that it is a matter of Ijtihad that scholars can dispute about.

4- As Muslims, we act on what is relevant to the proper values in relation to our environment. Now, with no slavery being practiced, the view of Ibn Hazm is the most relevant one to be implemented.

Almighty Allah knows best.