Formal legal equality/emancipation deserves credit for creating the legal condition of free labour. Capitalism deserves credit for creating the economic use of that free labour.

Then:

Some members of this working population subsequently achieved upward mobility and entered the middle class.

This gives you a division of historical contribution:

Development	Your attribution
Abolition of inherited unfree legal status	Formal legal equality/emancipation
Creation of legally free persons	Formal legal equality/emancipation
Availability of free labour	Legal transformation
Employment of that labour in capitalist industry	Capitalism/industrialization
Formation of a working population	Interaction of legal freedom and capitalism
Upward movement of some workers	Economic opportunities within capitalism
Some workers entering middle class	Upward mobility within the capitalist economy