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I think it's important to remember that one can disagree with their brothers and sisters in Christ about things that are not essential to the faith. For instance, there will be disagreements amongst Christians about whether baptism should be full immersion, or simply a sprinkling of water. This is not essential to the faith, meaning this does not determine whether a person is a Christian.
Something that is essential to the faith is that Christ died on the cross and rose again. If a person does not believe this they cannot be a Christian.
I think what these verses are saying is that each person should figure out the non-essentials of the faith on their own, and if their brother or sister disagrees with their viewpoint they should not let it divide them. To me, unity of mind means to come together with other members of the Church simply because they love the Lord and they want to worship Him together. Disagreeing on small things is healthy, and it opens minds.
I hope this answers your question. I decided to answer this question considering Christianity at large instead of through a Lutheran lens because it seemed to fit better with the verses you were asking about.
"But God has so composed the body, giving greater honor to the part that lacked it, that there may be no division in the body, but that the members may have the same care for one another." 1 Corinthians 12:24-25