Hi,
You have mentioned that Shemittah today is derabbanan according to many major authorities. Do Shemittah and Yovel apply in Israel today, and is each obligation from the Torah or rabbinic? If Yovel no longer applies, why not, and when did it stop?
Yovel is not operative today because the tribes of Israel are no longer settled in their designated territories.
Regarding Shemittah, there is a major machlokes among the Rishonim whether it is a rabbinic obligation today or a de’oraita. That means a Safek of Shmitah is either a Safek Derabanan or A Sfek-Sfeka De’Oraita. In practice, these are generally treated similarly, so there is little reason to complicate the matter further.