What is the difference between "Besamim" and "Samim"?
Ramban #1: Gramatically speaking, Besamim are edible (spices), whereas Samim are medicinal.
Ramban #2 (citing Rashi and Targum Onkelos) and Targum Yonasan: Samim is synonymous with Besamim. 1 However, since the Torah uses two different words to describe them, it is logical to say that Besamim are the more important spices, 2 and those with the more pungent aroma.
Ramban #3: It is also correct to explain that "Besamim" is the acronym of 'Bo Samim' (it contains Samim).
What exactly does the Pasuk mean when it writes "ve'li'Ketores ha'Samim"?
Ramban #1: The word "ve'Samim" is missing, and it is as if the Torah had written "ve'Samim li'Ketores ha'Samim". 1
Ramban #2 (citing Ibn Ezra in the name of Chachmei ha'Dor) and Rashbam: The two words are inverted, as if the Torah had written "ve'ha'Samim li'Ketores".
Ramban #3 (citing Ibn Ezra): "Besamim" goes on both the Shemen la'Ma'or and the Ketores ha'Samim. 2
Because the Ketores did contain some spices (Shiboles Nerd, Karkom and Konmon, as the Gemara states in Kerisos, 6a). On the other hand, the Torah does not need to mention Samim for the Ketores ha'Samim, just as it does not mention Shemen for the Shemen ha'Mishchah (Ramban).