I can understand churches that say they do not need to observe the law (except those discussed in the NT). And I can understand those very few Christian groups that say they need to observe all 613. What I don't understand is those who pick and choose, as if the law were a buffet that you could take just what you like.
You may worship on Sunday because that is the day Jesus supposedly rose, I get that, but it is not the Sabbath. The commandment is to rest on the seventh day. Unless you are doing that, you are not keeping the Sabbath. And as you admit, you don't even require that everyone rest on Sunday, so it doesn't even look to me like you are trying. I don't say that to be snide. It just really looks that way to me.
You seem to have a misunderstanding about Judaism, so please allow me to inform you of what we actually believe. In Judaism, LIFE COMES FIRST. To preserve a life, we not only CAN break a law, we MUST break any law that stands in the way of saving someone. This is why you have Jewish doctors working in hospitals on the Shabbat.
God does not change. As many of my Christian friends have said to me, "He is the same yesterday, today, and forever." It is also true that the covenant between God and Abraham is "everlasting." That too is unchanging, then.
There are universal laws that everyone in the world is to follow. They are pretty obvious like, Don't murder, Don't steal, etc. The Law that was given to Israel at Sinai included those laws, but had many others besides. These additional laws are ONLY for Israel -- you as a non-Jew are not obligated to keep them. So for example, it is a sin for me to eat bacon, but you can eat bacon to your heart's content and not sin. It is a sin for me to labor on the seventh day, but for you, you can go out and mow your lawn, and it's fine. So enjoy your lobster.