I'm still curious about how the Civil War will play out. I'm assuming boring old neutrality in the same fashion as Britain. Unless the Nobility tries something stupid. Assuming the nobility would still be in power after decades of being surrounded by Freedomland. Transylvania could be an possibly be an independent democracy or constitutional monarchy by 1850-1860 whenever the US splits. Not saying they'll take over Transylvania, but the numbers would be in Unions favor should conflict happen at that point.
In OTL, Mechanization of agriculture beginning in the 1830s led to a population boom and enough of a surplus to fuel a market economy and power urbanization. By the 1860s, Transylvania had about as many people as New York, which was at the time the most populous state.
The American public, on principle, would be opposed to the Noble-dominated feudal political system in Transylvania. Northern abolitionists would equate it with slavery, but Southerners would probably dispute the comparison since in this case the peasants are white. Since Transylvania is overwhelmingly rural and slow to mechanize, its economy will be weak and it will have little ability to trade with the US.
In the first years after the ISOT, Transylvanian nobles would probably become increasingly aware of American influence in their politics; the more reformist nobles would probably make frequent trips to the States to drum up political support, which would directly threaten the rights and privileges that the nobility had been working so hard to preserve. There would probably be an internal power struggle where the reformers would be forced to flee the country.
The attitude of the United States towards peasant settlement would probably change and shift depending on the political climate. Before 1830, US politicians wanted to set a moderate price on frontier land so that the proceeds could pay off the national debt. The idea of allowing non-English speaking squatters to settle there for free won't be popular; the easiest solution would be to allow Transylvanian soldiers to enter western territory for the purpose of recapturing peasants who entered US territory with significant debt (which, due to the system in place, is all of them)
Assuming the US maintains its nominal territorial claim over the area Transylvania occupies, the nobility will be keenly aware that the United States represents a threat to their way of life. They might side with the British in the war of 1812, seeking to acquire the rest of the Northwest Territory for themselves. The Szekely troops number in the tens of thousands, which is more than either the US or the British can raise in the early stages of the war. The Szekelys could probably march south and raze Saint Louis to the ground, but because the area is so unpopulated it would be impossible to provision them, so they would have to retreat. The US might be forced to recognize Transylvania's independence in the aftermath, but I think it might end up with no territory exchanged as in OTL. Britain would probably try to prop up Transylvania as a regional power in the Great Lakes to keep the US from controlling the area.
It's anyone's guess how Transylvania and the US will end up divvying up the North West Territory. The US would probably advocate free homesteading earlier than OTL in an effort to keep Transylvania from expanding and increase support for escaping Romanian peasants. As some Romanians flee westward to purchase land, others would gather in US cities and begin to form their own literary scene, which would in tie contribute to a Romanian Transylvanian national awakening.
In the 1820s, younger, more reformist nobles such as Miklos Wesselenyi will become more prominent. In the 1830s, greater mechanization on farms will lead to higher surpluses, which will allow towns to support market economies. Meanwhile, the US population beyond the Appalachians grows to rival Transylvania's population, which is still growing slowly. Increased trade leads to increased demand for luxury goods such as cotton, which they can get through trade with the US. Transylvania will find that a belligerent attitude towards the US is unsustainable and instead fosters a more positive relationship with the US. Urban craftsmen and guild members are attracted to American cities by high wages in factories, which weakens the power of the guilds and allows merchants and capitalists to fill in that gap.
As travel between the US and Transylvania increases, republican ideals will filter its way into the upper and lower classes; the question is not if, but when. In OTL, the attempt to abolish serfdom in 1848 led to a full-blown peasant revolt; Hungarian nobles tried to manipulate the situation to keep the serfs in subservient positions and the peasants revolted. My guess is that sometime in the 1830s, there will be some kind of Romanian awakening. Perhaps it could coincide with upstate New York's Anti-Rent war of 1838.
The end result of the Transylvanian Revolution may not be a full democracy; rather, I think it's likely that it would become a constitutional monarchy. In the early days of the switch, the Diet would probably elect a Prince to serve as the head of state, and they'd likely chose Gyorgyi Banffy, who was a prominent statesmen from an old and powerful family. Banffy was himself fairly reform-minded and would pass away in 1822. If the monarchy can retain good relations with the peasantry (which is not difficult; since the King is distant he often gets none of the blame for hardship) then the basic structure of the monarchy would likely stay in place. Romanian literati (of whom there were few) periodically proposed expanding the Diet to include proportional representation for Romanians, which would give them around 55% of the Diet; the nobility wouldn't allow that, since it would render them effectively powerless. They would be more likely to accept splitting the Diet into one house populated by landed nobility (the 250 or so already in the Diet) and one apportioned according to overall population and ethnic group, not unlike the UK's House of Lords and House of Commons.
By the 1840s, the US's population beyond the Appalachians will have grown to the point where Americans outnumber Transylvanians. Since Transylvania is relatively densely populated, though (it has the population density of New Jersey in 1840), it's not really suitable for settlement or homesteading, so the US doesn't have much use for its land. The areas of Indiana and Michigan that are primarily Romanian-speaking would probably be a contentious issue, as Romanian settlers there would seek to become independent from the US by the same logic that Texas became independent from Mexico. However, the US would not tolerate this and suppress any revolts, which would result in tense relations with Transylvania.
Transylvania's stance on slavery in the 1860s would depend on many factors, including their economic reliance on cotton. However, it's likely that they would oppose the practice on principle, drawing from their own experience with serfdom. However, whether that translates to full Northern support is uncertain. Since this is a US-internal matter, I think the official stance of Transylvania would be to remain neutral, but continue trading with the North.
Transylvania may or may not become a stop on the underground railroad; while it's conveniently located next to Kentucky and presumably not subject to fugitive state laws and so on, they could absorb a decent number of escaped slaves; the 100,000 that fled in OTL would represent just 5% of Transylvania's population, which would make them about as numerous as Roma. (speaking of whom, it's practically guaranteed that Roma would be more prominent in the US in this timeline)
By the 20th century, Romania would probably just end up as a fully independent country between the US and Canada, with Romanian, Hungarian, German, and English as official languages. It would lag developmentally behind the US, at least until the latter half of the 20th century.
