This is really lovely work!
The neckline on your cardi is fine IMO! Mainly because it will be worn with a top underneath anyway. But I follow why you feel that way, especially since you mentioned that the yarn wants to lengthen anyway.
Well, it looks wonderful on you, and you should be very proud!
The blanket is awesome! Such great work, nice even knitting! Perfect! I know how you feel about the border flipping over. I've had the same dilemma with scarves!
But I also agree with others that it won't matter one bit when the blanket is in use!
I've tried several remedies for this 'flipping'...like making the border wider by 3 stitches, but nothing ever cured the 'flip factor' because no matter what I did, the border lays next to st st!
Maybe (next time) don't work any border at all. When you're done, work an applied i-cord along the edge. It would nice to experiment with this first before committing to working the entire blanket without the 3-5st built-in border.
What does everyone else think?

Would an applied i-cord right next to the st st work better than the built-in garter stitch border?