*snip*

20+ years in the bike industry and keen hobbyist general 'engineer' & DIY framebuilder here... I feel your pain. Some bike stuff is weird and just wouldn't be acceptable in the areas / tolerances you normally work in!

The frame is definitely for the larger 41.8mm size bearing, but it is not designed to be a press fit in my experience. The reaming tool I have measures a hair over 42mm, for example.

In short, I'd stick the 'correct' bearing in there (41., and ream the head tube so the bearing surface is nice and clean. That should do the trick. You can buy just one of these bearings separately if you need, and all the other hardware you've got already will work just fine.

The 'next level' would be to get a nice quality headset, something like the one you linked to, with a top plate that actually is the correct height and has a seal - however, don't be alarmed if you have to spend some time tinkering to make that happen, there really isn't much of a 'standard' for how much bearings stick up out of top headset cups!