Getting back to question 1: It might delay the development of railway systems a bit because instead of being able to buy off-the-shelf in Britain, companies would need to order bespoke. THis will not matter too much in the early years and by the time the USA has its own railway industry, it will cease to be an issue, but in the interim, just before the Civil War, it will matter. The question is, how much? I don't know, but it is an interesting thought.
If they use Russian broad gauge, a lot of American rolling stock could be sold across the Pacific. Delivery's a doozy, and US industry is very competitive in pricing.
It could also lead to the (erroneous) conclusion that broad gauige is what allows things like pullmans, sleeping berths, dining cars and similar amenities. European standard gauge lines will not try to inntroduce them to the same extent. This might result in fewer long-distance luxury trains and more emphasis on medium-distance hops with railway hotel stays.