1) THE COVENANT OF THE RIVERBED
QUESTION: The Gemara relates that the Jewish people placed twelve stones in the Jordan River while they walked through the dry riverbed. On those twelve stones they wrote the words of the Torah. As they crossed the river, Yehoshua also made them accept upon themselves the Mitzvah of conquering Eretz Yisrael from the seven nations. He told them that if they do not accept that Mitzvah, the waters will come down and drown them.
Why was it necessary to do these specific acts while they crossed the riverbed?
(a) The MABIT (in BEIS ELOKIM) explains that it was necessary to put stones in the Jordan River (even though the stones would eventually become covered with water) for the same reason that a Mezuzah is placed on the doorway to one's home. The stones would remind them of the covenant with Hash-m every time they would enter Eretz Yisrael.
(b) The IYUN YAKOV explains that they needed to accept the Mitzvos which come with Eretz Yisrael while they walked across the Jordan because when one gives a present on condition, the condition must be stated before the present is given (RAMBAM, Hilchos Ishus 6:4). Once they already entered the land, it would have been too late to obligate them to accept the Mitzvah as a condition for receiving Eretz Yisrael. Therefore, Hash-m made them accept the Mitzvah immediately before they entered Eretz Yisrael, so that the gift of Eretz Yisrael would be given with that condition.
The Iyun Yakov seems to follow the opinion of Rebbi Yehudah ben Beseirah (Bechoros 55a) who maintains that the Jordan River is not considered part of Eretz Yisrael. Rebbi Shimon there argues and maintains that it is considered part of Eretz Yisrael. According to Rebbi Shimon, the reason why they needed to accept the Mitzvah of conquering Eretz Yisrael as they stood on the riverbed of the Jordan might be similar to the way Hash-m had them accept the Torah as they stood underneath Har Sinai. It was in order to show them that there is no life without the Torah. (See MAHARAL in introduction to Tiferes Yisrael.)
The reason why the Jewish people were given Eretz Yisrael is that it is the place which is most conducive to learning Torah (see Kesuvos 110b, Bava Basra 158b, and Bereishis Rabah 16:4). The gift of Eretz Yisrael is analogous to the gift of the Torah (especially according to Rebbi Shimon bar Yochai himself who rules (Berachos 35b) that one should not dedicate himself to working the land in Eretz Yisrael but should dedicate himself exclusively to the study of Torah).
2) KALEV'S PRAYER
QUESTION: The Gemara relates that Kalev went to pray in Chevron at the burial place of the Avos and to ask for mercy that he be saved from the evil scheme of the Meraglim. The Gemara says that Yehoshua did not need to go pray at the burial place of the Avos because Moshe Rabeinu had already prayed for him to be saved from the evil scheme of the Meraglim. The Torah says that since Kalev had a "Ru'ach Acheres," "a different spirit," he was rewarded by being given the city of Chevron.
Why did Moshe Rabeinu pray only for Yehoshua and not for Kalev?
(a) The simple answer is that a Rebbi's prayers are far more potent when he prays for a close Talmid. Yehoshua, who was a close Talmid of Moshe Rabeinu, would be protected by the prayer of his Rebbi. In contrast, Kalev -- who did not have the same relationship with Moshe Rabeinu -- needed to pray for himself since Moshe's prayer would not be as effective for him.
This is also why Kalev was granted a specific reward for his righteousness, the city of Chevron, while Yehoshua received no specific reward. Since Kalev had to struggle with his Yetzer ha'Ra and pray from the depths of his heart in order to reject the plan of the Meraglim, he was rewarded. Yehoshua never entertained any thoughts of joining the Meraglim because Moshe Rabeinu had prayed for him. Since Kalev's struggle was greater, he was rewarded. (See ALSHICH to Bamidbar 14:22; OR HA'CHAIM to Bamidbar 14:24.)
(b) The CHAFETZ CHAIM (Parshas Shelach) suggests a different approach. He says that there are two types of Tzadikim. One Tzadik protests loudly as soon as he sees anyone do something wrong. The other Tzadik does not say anything, but instead he waits for the opportune time to speak up and explain to the wrongdoer what he did wrong in order to influence him to do Teshuvah.
Each of the two approaches has an advantage and a disadvantage. The advantage of the first approach is that the Tzadik will not be influenced by the evil ways of the others. However, the disadvantage is that his words have less of a chance of influencing them, and they might even attempt to physically harm him in order to stop him from rebuking them. The advantage of the second approach is that the Tzadik's words are more likely to have an effect, but the disadvantage is that by remaining silent in the face of evil he may become desensitized to the sins that he constantly witnesses, until the sin eventually becomes light in his eyes as well.
Moshe Rabeinu knew that Yehoshua was the first type of Tzadik. Moshe Rabeinu did not need to pray that Yehoshua not be persuaded to join the Meraglim in their evil ways, because he knew that Yehoshua would protest loudly and reject them from the start. However, Moshe Rabeinu realized that he did need to pray to Hash-m to protect Yehoshua from the plots the Meraglim might make to harm him so that Yehoshua not foil their plans.
Kalev, on the other hand, was the second type of Tzadik, and therefore Moshe Rabeinu did not need to pray that he be protected from physical harm. Instead, Kalev needed protection from falling into the trap of the Yetzer ha'Ra and following the scheme of the Meraglim. One person can pray for another person only to be protected from physical harm, but not from being seduced by the Yetzer ha'Ra. That is why Moshe Rabeinu's prayer would be effective only to protect Kalev from physical harm, but not to protect him from his Yetzer ha'Ra. Kalev would have to fight that battle himself by praying from the depths of his heart in Chevron. This is the meaning of the verse which says that Kalev was of a "different spirit" -- he was "Echad b'Peh v'Echad b'Lev," what he said was not what he thought (Rashi). He told the Meraglim that he agreed with them, but later when he found the opportune time he displayed his true intention, to foil the plot of the Meraglim.