1) Your understanding of "means to an end" and "an end" seems to be correct, but I think you missed out a "never" in your explanation. It says you should never treat humanity as a means to an end. "whether in your own person or in the person of any other" is talking about the places in which humanity resides.
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SweetFreedom1)Does “Act in such a way that you treat humanity, whether in your own person or in the person of any other, never merely as a means to an end, but always at the same time as an end”mean "act/behave in such a way that you treat humanity, whether for yourself or for others, merely as a means/tool/way for a purpose, but always at the same time treat humanity as