According to a local quilt shop, if you change the pattern by 25% you can claim it as your own. (???)
As to the kit issue. I have made patterns that were from kits, because I loved the colors and the pattern of the kit.
I have also purchased kits that had one or two fabrics that I did not like in any way, so I used my own.
I believe kits are a convenience issue only, and you are certainly NOT obligated to purchase a kit to make a quilt that you love. If that were the case how many of us are already outside of "quilt law" by making patterns we see in magazines that also offered kits???
If you give credit to the designer I would say:
"Inspired by the quilt designed by Jane Doe."