Hmmm, I think the level of scaring would depend on the tailor, of course there will always be a bit but some might be able to reduce it more than others, i've never really noticed it with our tailor.
It looks too be fine on the shoulders, to me, I think the issue here is the construction more than the fit. The restrictive material means it doesn't have as much movement as the normal suit jackets for example so it will always feel tighter. I've tried on all the casual blazers that have come into store and I've had a bit more trouble getting into them than normal (especially with the arms) and I'm a pretty much perfect 36, once on they fit fine, so i've experienced pretty much the same issues as you.
The fit everywhere else looks good so I'm inclined to say keep the 40 and see how you get along, you can always have some small alterations done at a later date if you think it needs it, if you're only taking a tiny bit out you probably wouldn't notice any major marks, depends on how much you feel it restricts your movement. I think you might find the 42 is a touch too far the other way and it would be too big on the shoulder; you would end up having to do too much work on it to get it to fit the same, leaving a risk that you change the shape of the jacket.