Hi andras, some more information on your #1.

So when you do a blend between 2 closed edges like this for example:



Part of the controls that will come up is a "seam alignment point" that will let you adjust how the 2 closed curves are being connected together, it will show up as 2 small points like this:




If you do a the blend on your #1 case, you will notice that you do not have those points available. That's because the top edge there has a little tiny gap in it:






That's a big enough gap that the blending code does not consider that top edge to be closed.

The blending mechanism will only be able to put up an adjustable "seam point" for blending between 2 closed edges, when edges are not closed they can only be flipped to help with orientation, not "re-seamed".

How was the top edge curve created for your #1 case, was it from trimming your previous shape that you posted followed by Merge or was it from some other steps?

Whatever produced that top edge may need to be tuned up to "true up" the endpoints when there is a topologically closed loop being formed but the endpoints are slightly apart from one another.

- Michael