If the A-mighty could have an afterthought, then the conclusion of Parshas Tetzaveh would unquestionably be its paradigm. However, since Hashem cannot have senior moments and lapses, and never is remiss by not remembering, we have to find another explanation for the seemingly tacked-on Parshas Ketores at the end of this week's reading. The mishkan seems to be complete, the clothes of the kohanim are done and worn, the dedication days services are all described and then comes the seemingly "oops moment". We read about the construction of the golden mizbeach that will be placed in the mishkan and a brief sketch of the incense service, all of which seems a much better fit for Parshas Teruma which is already replete with details of all aspects of mishkan construction.
Whereas Rambam (Moreh Nevuchim) understands that the ketores is similar to a powerful incense candle which assures that the mikdosh does not smell like a butcher store, Chazal seem to add many deeper meanings as well. After all, Chazal describe that the aroma of the incense would be carried all the way to Yericho! Surely if the Jews of midbar Yehuda were to smell the ketores there should be another meaning in that as well. Additionally, the ketores emanated from the inner sanctum and brought wealth to the kohanim who offered it!
The broad and overwhelming presence of the ketores leads me to believe that it served as a reminder to anyone near the walls of Yerushalayim to stop and smell the fragrance. The all-embracing nature of the ketores bouquet begged each resident and every passerby to pause, take it all in, and appreciate the privilege and responsibility of having Hashem dwell among us. Similarly, that is why it is the capstone of the parshiyos describing the construction of the mishkan. It thus serves as a textual symbol of the role of the ketores, reminding us that even as we study the details of the mishkan, we should ponder the big picture and become mindful of our heritage and our ultimate enterprise.
There is very little in life that is of value that does not gain additional value added by the moments when one can look back and appreciate what we have accomplished.
If this understanding of the ketores is correct, then it may be the insight of the Seforno as he addressed our opening question. Seforno explains (and his words may shed light on Ramban's comment as well) that the incense service is the only service that celebrates Hashem's presence, as opposed to all other aspects of the mishkan which are there to bring the Shechina to the mishkan to begin with. That is why the ketores is described after we read about the construction of all the instruments and their successful consecration which indeed resulted in the settling of the Shechina in our midst.
In other words, the pensive pause that we have described, the "take it all in" moment, is indeed a moment of celebration and even more. For many, I imagine, the inescapable scent would be received as a divine hug; for the lonely and worn, the curious and the skeptics it would be an invitation to come closer, and maybe it was even perceived as Yerushalayim reaching out to them; and for some it would simply remind them of their core, their roots, and their ambition. But for all it was a soaring manifestation of the entrancing reality of having access to G-d from this very human, very flawed, and often painful planet.
A discussion of the incense altar any place but at the end would be totally misplaced.