Questions
What would be the Texan Position on the American Civil War
Texas remaining independent might just butterfly away the Civil War.
As others have pointed out, an independence Texas is likely going to have ties with Britain. Considering that, and the US-British disagreement over Oregon, I think the most likely results are an agreement with Britain getting Texas and the US getting all of Oregon, or an Anglo-American War in 1846.
If it’s war, the US has significantly higher population than Canada and Texas combined (which didn’t help the US defeat Canada in the Revolutionary War or the War of 1812), while the British Empire has significantly higher population than the US (which didn’t help the British defeat the US in the Revolutionary War or the War of 1812). It will probably come down to commanders – OTLs US had good commanders in the Mexican-American War, Britain’s track record, from the invasion of Afghanistan to the Crimean War is a lot more spotty.
Other considerations are there were existing border disputes between Arkansas and Texas and between Texas and Mexico. It’s entirely possible that instead of a Mexican American War in 1846 over the disputed area, we have an Anglo-Mexican War. Unless, the British have as good of leadership as that of Winfield Scott and as good of scouting as that of Robert E Lee, they’ll have a harder time and lose more to diseases.
Within the US, having Canada to the north and British influenced Texas to the south will probably encourage “we must all hang together or surely we shall all hang separately. Southern expansionists will want to conquer Texas, so if the South every does attempt secession, Texas is unlikely to support them, and might even attack the CSA as an ally of the USA.
Would Mexico Hold California
Not after gold is discovered, there just isn’t enough Mexican population there. This is especially true if there’s an Anglo-Mexican War going. The US might seize it, though unlike OTL, they’d have no interest in Arizona-New Mexico – an intercontinental railroad can’t take a southern route with Texas in the way. Other possibilities are California becoming a British colony, becoming independent, or being annexed by Texas.
Would Alaska still be russian
Likely not. Russia still needs the money and if the US controls all of Oregon territory, there’s more reason for the US to buy it than in OTL.