559 forks usually allow a lot more tyre clearance than 622 forks. Traditionally, 559 MTBs could be fitted with skinny 700C road wheels and tyres, sometimes with mudguards.
You are changing both front and rear wheel, so the difference in radius between 559 and 622 is irrelevant. The whole bike moves up and down, just like changing the tyre width.
The only real difference is the difference in fork length. Your assumption for that is 14mm, a very small percentage of a tandem's wheelbase. I will let you run the numbers yourself to find the angle difference. The very rough rule of thumb for solos was 1 inch = 1 degree.
A good designer will use different fork rakes for 559 and 622 forks and for different cycle types. You have assumed the same fork rake for both but you are talking about a custom tandem. Unless you are using carbon forks, you get to choose the amount of rake on your steel fork, so you can choose whatever trail value you want. Generally tandems steer better with about 10-12mm less trail than a solo with the same head angle. Whether a tandem runs a steeper head angle or a more raked fork (or a combination of the two) than a solo is up to you.