As for a North American Union, the simplest period way to accomplish it would be to allow the USA to unite the continent by annexing, or getting cores on, all of it with appropriate events/decisions. E.g. modified versions of Manifest Destiny and Ostend Manifesto decisions that award the USA cores on Canada, Mexico, the UCA, and the Caribbean, and a modified version of the Texas annexation event that allows the USA to annex North American states when they are in its sphere and relations are good enough. Since there were historical calls to annex those territories in 19th century US mainstream political discourse, I think this would be highly appropriate.