another episode of endless rants about blends and fillets and other geometric dysfunctions of rhino.
davide if you really want to force those edges into position then by all means dont use history so close on the edge of abilities which require 2 surfaces to intersect at a new defined extended boarder.
no wonder this rips open and not only because of blend or history, i am not sure if there are many applications which will do that automatically for you. maybe there are some and i also dont mind having it all go in one click.
anyway in case you want to have an alternative option, i built something similar (could not open your version 6 file) -> first find the perpendicular 2 tangents inwhich the hard edges meet each other and section the 2 surfaces with it where you need it.
use BlendCrv or blend then Sweep2 and hey
then be happy that it took only 2 minutes