What good did Moshe promise Yisro now that he did not promise him in Pasuk 29?
See Ba'al ha'Turim.
Rashi: As the Pasuk states in Shoftim, 1:16). This land was given to Yisro's descendents until the Beis-Hamikdash was built, at which point it was given to the owner whose land was appropriated to build the Beis-Hamikdash on it.
Even though he is not mentioned there, but in other places in Tanach - See for example, Yirmiyah, 35:1.
Ramban: Which they later received, even to the point of being obligated to bring Bikurim from it and reading the Parshah.
Did Yisro remain with Yisrael, or not?
Ramban: Yisro remained with Yisrael and entered the Land together with them. 1
Seforno: Yisro's sons did indeed remain with Yisrael and enter Eretz Kena'an together with them. 2