Texas really isn't implausible, unlikely, but not implausible. If Houston dies from a heart attack or some such event Lamar will be the main leader in Texas politics. Texan politics revolved largely around individuals, not parties. And I think if he wanted to, he could have kept Texas independant.
Canada is possible too. If Canada can win in 1812, and takes control of the Northwest (Northwest at time anyways) a more northerly Oregon trail could be established firmly entrenching Britain/Canada there too. And while his state won't oppose America soon, it could by the mid 20th Century.
Would not do alone.
Things are calme in the 1830s and 1840s, much of the 1850s. Hovewer, things changes in the latter half of the 1850s.
The French conquers Mexico, with Texas opportunistically siding with them, they face guerilla warfare hovewer. In exchange for helping Maximilian maintaining control over Mexico, the Texans are granted California(1861) and soon after gold is discovered there (1862).
The south still attempt to become independent (1864) but Britain side with them this time, parts of the US North-East becomes part of Canada.
1867-1869, France intervenes in Spanish civil war and put in power pro-French regime there.
Napolean 3 attempt expansion into Germany but end up uniting many German states behind Prussia and is defeated, revolutionnary war in Mexico begin as a result, Texas expand borders south once more in 1875-1878 period.
The USA militarises under Lincoln, when Britain declares war to Greater Prussia over the seizure of Istanbule in 1881 by Greater Prussia and its balkanic allies, the US declares war in support and is assisted by a Russian expedition force.
CS forces are smashed quickly, gas and burning liquids are used to clear trenches and defensive positions, the British launch a serie of naval attacks on the US east coast.
In 1882, war with Prussia ends with Britain being granted its share of Constantinople and the harbour there(remant of the ottoman empire is less independent from London than Australia and India, becomes TTLs General Governement of Poland in the following decades), France and Spain declares war to the USA for an alliance with Britain.
The USA win against the CSA but the greater strenght of the Canadian/French/British empires eventually triumph and even more territories are gained in the North East. Remnants of the CSA prefer Texan influence over being a British puppet.
WW1 start with the USA discovering the Haber process(different name TTL), sign a non-agression pact with Texas (that takes control of the deep south) and attack Canada in full strenght, the canadians are beaten before the French-British-Spanish can come to their assistance.
US east coast faces a serie of attacks but the British fail to gain a foothold back on the continent.
The USA, once confident the allies are not a threat, demand return of all confederate territories to it and launch a massive attack before Texas gives its response.
The attack meet with prepared positions hovewer, allies comes to the assistance of Texas, while Russian Imperial forces strikes the US in the west, Cossacks rampages through much of the mid-west.
Finally, revolutionnaries begin strikes that breakes US ammunition production, major US offensive fail as a result, more strikes results from the ensuing defeats and the US navy personel begin revolting against the governement, junta is overthrowed and replaced with what historians would later call the
The Pittsburgh Republic.
The USA is humiliated and reduced to a second-rate power by the peace treaty, France becomes enough confident due to the american campaigns to attack eastward, striking through Belgium (past fortifications in Alsace-Lorraine) and seizing the Rhineland.
Spain and Britain, seeing the sucess of the French and fearing a French defeat by Berlin and its allies would grant domination over the continent, joins.
Texas conserves the deep south, Amendement on religious tolerance(unless said religion is a source of civil disorder) means the Deseret state joins Texas.