So I had to rip the whole side out. Once I've seen directions on the Web about how to rip stitches easily. I didn't believe it would work well but now I decided to give it a try.
You cut every fourth or fifth stitch with the seam ripper on one side. On the other you can just pull the thread away. I was really surprised how easy it was. The stitches let go of the thread instantly.
That's for the ripping. The best to do then was to sew the pike in hand (which I had wanted to do as last). I folded it like a jeans pocket and attached with never disappoining invisible backstitch. This way I could be sure the gore wouldn't move anymore.
I started to sew exactly at the point I had finished at. Now it's a lot better:) However I wish I found a method of inserting a real triangle smoothly, but until then this does.
Now I have the petty almost finished, my friend just has to come and let the skirt shortened on herself. I'm quite happy that this dress doesn't require many trials except for this one. I like this work! :-)