For example, in order for Apple to support business operations in a non-US country, they need to establish the infrastructure and comply with all of the local regulations/laws. There is maintenance of the online store and brick and mortar stores. These costs will then be spread across all of the products that are sold in that country. If the country has a smaller sales base, then the infrastructure costs per unit are higher than in a country with a larger market. This results in a higher unit cost in smaller markets.
Yes, but that infrastructure also costs money in the US. Apple has a large number of stores in the UK - their infrastructure and logistics here must be pretty efficient.
The largest difference is probably due to the different trade and consumer regulations in Europe. If the standard warranty in the US is 1 year, in Europe it is effectively at least 2 years.
There's support under European law to take a product back for repair if it fails within what you'd expect to be a 'reasonable life' for that sort of product. That can extend well beyond the statutory two years for something like a computer which you WOULD reasonably expect to last longer.
These laws must add a quite significant extra cost to those doing business in Europe - and from the consumer's point of view it's worthwhile comparing the higher price of Apple kit here, compared to the saving you may be making on not really needing to buy Applecare.