Immigrants would lead to higher unemployment
You need to specify who the following leaders are if you want it to make it possible to answer your question.
William H. Sylvis was the leader of the National Labor Union.
Yes
It convinced leaders not to make laws addressing issues such as child labor and pollution.
Most people find its easier to follow rather than lead There are two types of people natural born leaders and natural born followers
Immigrants were willing to work for relatively low wages.
This depends on how you want to apply the term. In a strict sense they did not have a dual labor market, even the 'shaman' and tribal leaders took part in the lower forms of work as it was the responsibility of all (an idealogical socialist structure).
labor laws, opposition from corporations, blacklisting of leaders
Because the US doesn't lift a finger , Our owners want cheap labor and unions are not part of the long term plan.
A bunch of labor union leaders and he created the Congress of Industrial Organization. The organization ended up getting very popular and large in size.
Collective bargaining