As the first question was thoroughly answered, I'll just add my two cents to the second.
Honestly, I can't see the end result changing much if Oregon went entirely British. American expansion from the time after the War of 1812 until the American Civil War revolved around the idea of a balance of power between the slave states and territories and the free states and territories. Without that huge chunk of territory which would obviously not admit slavery if/once the land became solely American, there would not only be a reluctance to directly annex the entirety of California (I imagine there being a proposal to leave the land south of 36 30, Virginia's southern border (nothing to do with the Compromise of 1850), Mexican, but there would definitely not be a push to annex further land following the Treaty of GH (IE. Gadsen Purchase or more) for a number of reasons. For one, that's more southern territory. Two, the north does not have Oregon (not only a geographic principal but a principal of morale as well -- likely embarrassed about the land being in British hands) and thus would be weaker, leading to more legislation for there to be but one trans-continental railroad in none other but the northern, free, territories despite the best southern efforts to create a second line. After all, the land annexed by Gadsen was for the sole purpose of opening a route for a southern line.
To address the question in a different manner, the fact of not having any bit of Oregon would not provide incentive to gain land elsewhere to compensate, but would instead provide disincentive to annex any more southern land than that which was necessary.
If you want America to gain all or more of Mexico for a scenario you are planning to write, have America either do overwhelmingly better in the War of 1812 or have America fight harder for the 54 40.