Hmmm.....Very interesting ideas, for certain. How about a preliminary layout such as this:
1840: RRG secedes from Mexico, and in a treaty with Texas agrees to a Rio Grande Border in exchange for national recognition, settlers, troops, and aid for secession from Mexico. The capital moves from Laredo, TX to Monterrey, RG. More than 12,000 Americans and Texans move to the new land.
1840-Feb 1841: the RRG signs a treaty with Mexico setting their border, contemporaneously with their treaty with Texas setting up a Texan Embassy in Monterrey and a RRG Embassy in Austin. Free land is offered to any American who wishes to settle, and word is also sent to colonization societies in Europe.
1840: President Van Buren begins a revitalization of Alexandria, DC, with a number of his cabinet officials taking residence there, along with a number of foreign dignitaries as well. Alexandria begins an economic expansion that leads it to side with remaining in the district to this day.
1840-46: RRG, eager for settlers, passes similar legislation to Texas encouraging settlement, bringing in German, Swedish, Polish, English, French, Italian, Greek, and Russian settlers in varying numbers. In all, around 45,000 immigrants join the nation.
1842: Webster-Ashburton Treaty settles the Lake of the Woods, and the Maine / New Brunswick border, but defers the Oregon question for later (can Britain be otherwise occupied, or increased American presence press the Brits to not want to handle this now?)
1846: Mexican-American war starts with Mexicans firing on a contingent of US Troops along the RRG border. Secessionist movements all along northern Mexico had dogged Santa Anna, and the war was his shot to retake the RRG and Texas and expel the Anglos. The war lasts two years, and results in Santa Anna's defeat, and additional secessionist movements bringing in California, Nuevo Mexico, Chihuahua, Sonora, Baja California, Durango, Zacatecas, Sinaloa, San Luis Potosi, and the portion of Veracruz giving a coastline to SLP; the US military captures Mexico City and forces the treaty on Mexico, but they do pay $24 million to Mexico for the lands. Jefferson Davis' bill to annex the RRG wins 44-11. The Flag has 32 Stars (having added Texas, Rio Grande, Iowa, and Wisconsin)
1848: RG and TX are annexed into the US; this has an effect on the northerners, who press for the 54° 40' border as a counter the entire time. The British cede the land for little cost at the time, given the more pressing European matters at hand.
1850: The Compromise of 1850 brings California in as Free, organizes the South California Territory as a slave territory with its capital at San Diego, Sonora territory, Durango Territory, and Utah territory deciding via popular sovereignty. New Mexico Territory will allow slavery.
1862: Columbia Territory is split from Oregon Territory and Washington Territory.
1889: Washington becomes a state
1896: Columbia split into north and south Territories
1902: North and South Columbia join the Union
This might very well need tweaking, so feel free to tweak the specifics here. Let's say that the US gets the Mexican territory, and also all of Oregon country. Would it then be likely to get a treaty with Britain for a straight line up to the Arctic, thus bringing the Yukon into the US?