I agree fully that immigration must be brought under control. How is all of this that you describe affecting the switching of language from Spanish to English?
If you consider the urban areas where most of the immigrants in the 1840-1920, especially the non-Germanic immigrants, settled, they were patchworks of neighborhoods of differing ethnicities. A common joke in many cities was that a bridge over a creek or rail yard was the largest in the world because it connected Israel with Ireland (or whatever combination existed). The political world was often controlled by Irish Catholics for whom Gaelic was a distant memory and the business community was largely run by Protestants of British colonial ancestry. When a (say) second or third generation Lithuanian American wanted to leave the steel mills or coal mines, he had to learn English and acculturate as thoroughly as any WASP or Irish American.
The 45 year hiatus on immigration from 1920 to 1965 accelerated Americanization. When new arrivals ceased coming from Hungary, for example, there was a lessened connection to old country ways. First generation arrivals became older and passed away. Upward mobility and intermarriage eventually turned ethnic differences from vital matters into quaint cultural artifacts. A present day joke in places like Chicago is that St. Patrick's Day affords people whose forbears came from all over Central Europe an opportunity to become drunk and foolish.
This pattern of Americanization is what Hispanics should experience. Until about 20 years ago, they were on the same path as the Norwegians, Basques, Portuguese, and many other groups had taken. With massive Hispanic immigration, combined with more Spanish language friendly governments and even private sector employers, the need to learn English and assimilate declines. Furthermore, as whole swaths of the United States become Hispanic, it will be possible for upwardly mobile Hispanics to remain largely Spanish speaking even as they become doctors, lawyers, and business owners. If politicians named Gomez, Barrera, and Jiminez replace ones named Perry, Hutchison, and Cornyn, Spanish will become the language of state and local governments, and the Federal government in the Southwest. Consider what happened in Quebec in the mid-1900s. As French Canadians increasingly outnumbered their British descended neighbors, the Quebec provincial government effectively eliminated English from government and outlawed its use in most of the private sector.
In the end, it is a matter of numbers. If immigration from Latin America is ended or at least reduced to a trickle, there is a chance that assimilation will once again work. If not, we will see the nation split by language and culture.