Hi there. Houses out on remote flatlands or prairies would have been made out of sod prior to railroads/if you were not wealthy (most people weren't). If you had a lot of money, you could probably afford to buy materials and wagon them out to a remote place. When railroads came around, it became a lot easier to get materials around, and you started seeing wood houses (and later, pre-cut/pre-fab houses) pop up across the more remote parts of the frontier. But prior to that, houses were built out of what was readily available in that region. That meant sod on the plains, wood in Washington state and oregon, adobe brick and wood across the mojave desert (nevada, arizona, etc). Very few settlers would construct out of brick. If it was wood or sod, yes, it's going to burn pretty easily.

The Pony Express WAS a transcontinental mail system. It could get mail from the east coast to the west coast in 10 days. There was another service at virtually the exact same time, called the Butterfield Overland Mail service, that could also cross the entire country. The year for both of these is about 1860. When the pony express folded, it was sold to Butterfield. Mail in the wild west was delivered to post offices, not to individuals. An expansive mail service didn't come around until after the civil war, and even then, you could only get home delivery if you lived in a city big enough to generate revenue enough to pay additional mail men. If you lived in a remote place, you still would have to huff it to the station. So mail became efficient if you lived in a city after the war. If you didn't, it didn't become efficient until after 1900 when the country was getting more populated and air mail started.

What years were big for western emigration before of the gold rush? Honestly, not many. So california became a state in 1850 and the gold rush was 1848. There's not much time there. Nevada became a state in 1864 to fund the civil war (union army wanted gold in them thar hills), but there was essentially NO ONE there. In 1890, the entire population of nevada could fit in a baseball stadium! So sure, you had some folks heading out west, but there were never significant emigrations until formal statehood was declared in each state and homesteading started in 1862 (you should look into the homestead act, as this will help a lot with how populations grew across the frontier).

Were there a lot of highway men? No. There just weren't. There are some infamous ones, but cooperation was much more common on the frontier. When you don't know where your next meal is coming from, and you live in the elements, and at constant threat of indian or animal attack, you just weren't that likely to duel someone in the street or try and rob an armed coach. Some people did it, but definitely not "a lot". Indian attacks were largely against the railroad, and a lot of poor chinese and irish laborers took the brunt of that rage on behalf of the big railroad corporations.

What routes would have been taken? That's honestly a tough one. This country was criss-crossed with old stagecoach trails from the spanish and french before western states became part of the USA. Some of those trails became part of the official transcontinental routes congress ordered (basically a route from atlantic to pacific). Here's a really cool map that shows you the absolute labyrinthine system of stagecoach trails in 1847. Continue to use the library of congress' online map database for more great maps of old trails as I think that will really answer your question. Before there were official "routes", someone heading out west would get their hands on a map like this and head on out (they would not have been able to get a map as high quality as this).

https://www.loc.gov/resource/g3700.ct003227/?r=0.114,0.289,0.761,0.372,0

I hope that helps!