Blowing shofar on Shabbos - issur d'oraysa?

“V’ha’avarta shofar teru’a bachodesh hashevi’i b’asor lachodesh; bayom hakippurim ta’aviru shofar b’chol artzechem” (25:9)
Rashi explains the double instruction to blow shofar on “the tenth of Tishrei” and “Yom Kippur” as permitting shofar to be blown only on 10 Tishrei which falls on Shabbos, but not if Rosh haShana, 1 Tishrei, falls on Shabbos. Ramban immediately attacks Rashi for suggesting that tekiyas shofar is prohibited on Shabbos because of a pasuk in chumash – the gemara tells us that tekiyas shofar is prohibited only because of a gezeirah derabbanan lest someone may carry the shofar on Shabbos. How could Rashi suggest that this is a din d’oraysa? The simple explanation of Rashi is that he is citing an asmachta – the law is based on a din derabbanan which has a textual hint as a mnemonic. However, this seems a bit difficult – if an asmachta is just a mnemonic device, then how does it solve the pshuto shel mikra question of the double-wording of the pasuk? (See Ra’avad Mamrim 2:9 on asmachta). The Chasam Sofer writes that Rashi indeed meant that tekiyas shofar on Rosh haShana is prohibited min hatorah during a Yovel year (which is the context of the pasuk). The Ramban al haTorah writes that although the specific actions of doing business and many other dinei derabbanan are not formulated as issurim on Shabbos by the Torah, if one spends the day engaged in weekday activity, one has violated an issur d’oraysa of “shabbaton”. Shabbos must be a day of shvus, of resting, and destroying that spirit of the day even within the rubric of Torah-permitted activity is itself an issur d’oraysa. During the Yovel year, the blast of the Shofar served a business purpose – it announced the freedom of slaves for that year (according to the Rambam, the 10 days between Rosh haShana and Yom Kippur were celebratory days after which on Yom Kippur the slaves were completely free). With respect to Rosh haShana of any other year, Rashi agrees that the issur of tekiyas shofar is derabbanan. However, in consideration of the business implications – freedom of slaves, return of property – of the shofar blowing of yovel, Rashi held that tekiya of that Rosh haShana is an issur d’oraysa.