I thought the sealed system would prevent oil from tumbling out the caliper-side port, similar to how I can use a straw and my finger to lift coke out of a glass. But it doesn't seem to be the case here
I had similar experience with Magura brakes — just shutting the lever-side port closed did not stop oil from leaking from the caliper body's opening.
To undo the bolt attaching the brake mechanism to the fork, you need a Torx 25 star-shaped bit.
I had one in my toolbox, but they are readily available at DIY shops - just google ‘torx 25 bit’
There are two bolts on the brake mech, but it is only the bottom bolt which screws into the shoulder/boss on the fork, I didn’t touch the top bolt.
Removing the bolt ...