Rail Pass Help for travel in Switzerland & Neighboring Countries

My guess is that it would be cheaper to buy advance purchase (discounted) tickets for your 3 trips outside Switzerland:
1) Swiss border (Basel) to Köln (Cologne)
2) Köln to Amsterdam
3) Amsterdam to Düsseldorf
Düsseldorf HB (city centre) to Düsseldorf airport would be a local ticket.

To confirm this guess you need to look up advance purchase fares on the DB (German Railways) website: https://www.bahn.com/en/view/index.shtml

Eurail Passes are hardly EVER a good idea.

I usually would not buy two separate passes and would more likely be comparing the 3-country Select pass. It might loose in comparison to the Swiss Travel Pass. That's info I don't have and a can of worms we may not need to open. I accept the premise that you've chosen the Swiss Travel Pass.

I also assume that you've already bought flights departing Dusseldorf and can't fly from Amsterdam.

Advance-discount train tickets for your 3 trips outside of Switzerland will be cheaper than the 4-day, 2-country Select pass currently at $270 per person. But the pass is cheaper than buying on the day of travel for the faster trains that I think you'll want and gives you an extra day to play with. So you're mostly deciding between booking ahead or staying flexible.

I just happen to have in my drawer a ticket for Basel-Cologne-Aachen which I booked in May for next month. It's for First Class and it cost €43,90, just to Cologne would have been €39,90 - about $47. Obviously second class would have been cheaper (I think the price just to Cologne was €29,90). Booking today for the same trip on my travel date is around €20 dearer and on the day it will be 4 times dearer.

My tip for that ride is book on train EC8, it departs Basel SBB at 12:20 and arrives Cologne at 17:05. And First Class has a glass roofed panorama car.

But until October 7 the line is blocked and there are no direct trains.

Thanks for the help. To your point Steve, in booking tickets from Basel to Cologne, it shows a hazard sign and says that there is a disruption. Does that just mean it takes longer to get there or is the train route unavailable?

It means that until at least sometime in October, the train will stop in Baden-Baden and you have to get off and board one of the waiting buses that will take you the few miles to Rastatt where you will get off the bus and board the waiting train to continue on to Cologne. The rail line between Baden-Baden and Rastatt has been completely severed by a construction accident and repairs will not completed until October (we and Deutsche Bahn hope).

Current schedules account for this.

Reddog, there is currently one IC/ICE train per hour north from Basel to Germany, all other IC's and ICE's are cancelled (local trains are not affected). It departs Basel SBB at xx:13. It is nominally destination Frankfurt, but it will actually be train+bus+train. At Karlsruhe or Frankfurt you connect for other destinations.

I was at Basel SBB yesterday, there is a big red notice at the bottom of the departure board explaining this, and it says the situation will continue until 7th October.
Personally I would consider that date "provisional".

Edit
Information about the trains north from Switzerland (in German): https://www.sbb.ch/de/fahrplan/bahnverkehrsinformation/streckenunterbruch-rastatt.html
and details of the timings here: https://www.sbb.ch/content/dam/sbb/de/pdf/fahrplan/bahnverkehrsinformation/Linienplan_DB_Basel-Rastatt.pdf

There are NO reservations on this replacement service.
Unfortunately this happened at the worst possible time. Of the other two main routes from Switzerland to Germany:
Zürich–Schaffhausen–Singen–Stuttgart is closed due to (planned) re-building until 5. September 2017
Zürich–München is closed fro the same reason until 6.September 2017

Thank you Sam & Chris for your help.