How accessible are the many German-only expansions for Catan?
Allthough there have been numerous expansions and scenarios that were never published in English, it is still quite possible to acquire these scenarios in the US, along with reliable English translations of the rules. I would recommend starting at BoardGameGeek.com. That site can be a bit of a firehose for newcomers, but if you have a bit of patience, your efforts will pay off. 

After creating a free user account at BoardGameGeek, use their game search function to search for the scenarios and expansions that you're interested in. Each expansion/scenario will have its own information page. These information pages are big part of what makes BoardGameGeek.com such a fabulous resource for those of us interested in boardgaming as a hobby. 

In addition to basic game information and discussion forums for each game, the game's information page will also have two sections that will be of particular interest to you. First is the "Buy A Copy" section. This is where you can usually find used copies of the game for sale. These copies are generally supplied by individual hobbyists who set their own price and choose where they will and won't ship the game to. While many of the games you are asking about were only available in Europe, it's quite possible that the people who have listed their copies for sale on BoardGameGeek will be willing to ship them overseas. 

The second section of interest is the "Files" section. Here you will often find translations for rulebooks of games when they haven't been officially provided by the game publisher in a particular language. A quick search led me to an English translation of the Atlantis expansion this way, as well as an English translation of the Alexander and Cheops historical scenario. I also saw numerous copies of the Alexandar and Cheops scenario for sale by sellers in the US, all of which were asking $25 or less (plus shipping, I assume). 

You will need to create an account before you would be able to purchase from any of these sellers, but I suspect you'll find it worth your while. 

You pointed out that your list of examples was not exhaustive. I'd like to suggest that you try a game search at BoardGameGeek.com using only the keyword "Catan"; I think you might be surprised at just how much stuff has been made available for that game. Have fun!