And they have done it by adjusting the head tube angle, not the fork, right? I’m trying to understand how the geometry is working:-)

Marin have slowed the speed of the steering marginally (4mm extra trail) to compensate. The shorter stems will speed the steering back up a little, providing a sense of more steering input at the bars.

Yes you can. The only issue is that it exceeds the rear derailleur capacity, meaning your derailleur won’t be able to handle the 26t with the lower cogs on the cassette. Not a huge deal if you’re wary of what’s going on up the back. More info here: https://www.cyclingabout.com/wider-gear-range-road-shifters-gears-for-easier-hill-climbing/

I was wondering if you could just swap the inner 30 t chainring for a 26t or even 24t instead of replacing the whole crankset?

Interesting trend, it also helps on toe overlap! But do Marin and others change the steering in other ways (trail and rake) to compensate for the shorter stem or is the effect of stem length on steering not relevant?

The long reach is actually compensated with shorter stems to provide the same bike fit. It’s a trend that is now standard on mountain bikes, but is becoming more popular on gravel and touring bikes. Also, don’t worry about that diagram – it’s wrong in more ways than one!

Speaking of the T1, it’s on sale for $1200 now and it has Tiagra parts and a $150 Tubus rack, assuming they still have a 2017 in your size. It’s on the sporty side as touring bikes go. Mine came with 36h wheels this spring even though the spec says 32h. I don’t understand the 32h wheels, like this Marin, on touring bikes. I replaced mine with 40h ones anyways.