Thanks everybody for your input. I will hopefully get some update from the builder soon and possibly post it here.
The builder is from Russia, so he was apparently unable to source the specific crown although he had used the same one before. Well, this was probably the first alarm that I should stop.
So several things worry me here: strength of the crown - if too much material is removed (he apparently removed more than pictured in the first post). Inward-sloping shoulders give me the impression that it possibly could lead to greater forces being distributed to that area, but I'm no engineer, so I can only guess.
Aesthetics - I didn't expect it to look that awkward. It could look even more so if more material is removed, so we'll have to see.
The benefits of shorter fork in this case - not sure about this after retrodirect/glowbike's advice. I have strong attention to such details, and this detail is giving me headache, knowing that the fork is not as originally designed for the frame. Even knowing that the front of the top tube will go down 5 mm would probably give me some peace of mind. I appreciate your advice very much, though.
Well, if this doesn't look too good after the builder sends me the progress, I will have to propose him some other solutions - such as acquiring the original crown (sending it to him somehow) and doing it like he originally did. After all, he built the frame too. Otherwise I'll have to find another builder or just give up on this.
I admit that this was a rushed decision, going with this version of crown, since he lead me to believe that he can achieve the same geometry with it, but it turns out that it won't be so straightforward, and some compromises would be included.
EDIT: Well, here what he sent me. How does this look, everyone? I don't like this at all, so will bail out of this option. We'll see if the builder is willing to reconsider other options.