These are the names of the men whom Moshe sent to spy out the Land; and Moshe called Hoshea son of Nun, 'Yehoshua'.
By adding the letter י' to the beginning of the name, Moshe changed the name from שעַהו to יהושע. Rashi quotes the Midrash (במדב"ר טז ט) who refers to this name-change and says as follows.
ויקרא משה להושע בן נון יהושע. מה ראה משה להוסיף על שמו של יהושע יו"ד. כיון שראה משה אותן שהיו רשעים, אמר ליהושע, י"ה יושיעך מן הדור הזה. - Moshe called Hoshea son of Nun, 'Yehoshua'. What did Moshe see that he deemed fit to add the letter yud to the original name of Yehoshua? When Moshe saw that those men were wicked, he said to Yehoshua, "May Hashem save you from entanglement in the evil designs of these people".
It seems that Moshe's idea to add a letter to Hoshea's name, was that by adding the letter י' from Hashem's Holy name י-ה-ו-ה to Hoshea's name, it would give Hoshea a spiritual protection, which would 'save him from the entanglement in the evil designs of the spies'.
We need to understand however, why then did Moshe pray to Hashem - on Yehoshua's behalf - addressing the Holy Name of י"ה, which would infer that he was utilizing the first two letters of Hashem's Holy name י-ה-ו-ה, when all he was actually utilizing was the first letter of that Holy Name? Furthermore, if all Moshe contemplated was that by giving Hoshea a letter from Hashem's name it would offer him a spiritual protection, why then did he need to supplement that name-change with a prayer on Hoshea's behalf, beseeching that "May Hashem save you from entanglement in the evil designs of these people"?
The Zohar (קנ''ח ע''ב) tells us as follows about the people whom Moshe chose to spy out the Land.
...ה וכו' כֻּלָם אֲ נָשִׁים אֲשֶׁר שָׁלַח מֹשֶׁה וכו', אֶלָא אָמְרו רֵישֵי דְיִשְׂרָאֵל הֲוו זַכָּאִין הַוו כֻּלְהו, אִי יֵיעַלוּ יִשְׂרָאֵל לְאַרְעָא, נִתְעַבָר אֲנָן מִלְמֶהֶוִי רֵישִין, דְהָא אֲנָן זָכֵינָן בְּמִדְבָּרָא לְמֶהֶוִי רֵישִין, אֲבָל בְּאַרְעָא לָא נִזְכֵּי. - The Passuk says, 'Moshe sent them forth from the wilderness of Paran... they were all distinguished men and heads of the Children of Israel'. Initially these spies were all righteous men and leaders of the Jewish People. But they said, "If the Jewish People will enter the Land, we will no longer maintain our leadership, as Moshe will appoint new leaders over the People. For although we were worthy of leading the Jews in the desert, once we enter the Holy Land, we will no longer be worthy of this leadership".
The Kabbalists write that the Holy Name י"ה, which is one of the Names of Hashem, does not dwell upon one who is conceited, for one's conceit, so to speak, takes the place of that Holy Name. This is alluded to in the fact that the numerical value of י"ה, fifteen, is the same as the numerical value of גאוה, conceit. Consequently, because the spies that Moshe sent were flawed with conceit, as is demonstrated by the fact that they sinned solely in order to protect their leadership, this Holy Name was not able dwell upon them.
When Moshe added a letter to Hoshea's name, his intention was that by putting the letter י' at the beginning of his name, his name would consequently read י"הושע, inferring that the Holy Name י"ה dwelled upon him, for Yehoshua was different than the other spies; for he was humble and was thus indeed worthy of having that Holy Name dwell upon him.
In view of this we can answer both questions which were originally posed. Indeed, Moshe only added one letter - י', but his intention was that by adding that one letter, it would read out the two letters of the Holy Name - י"ה. Moshe did this in order to infer that the Holy Name of י"ה, dwelled upon Yehoshua. By inferring this, Moshe hoped to arouse Hashem's Mercy to save the humbled Yehoshua from the conceited idea of the other spies. Thus, the name-change and Moshe's supplemented prayer were actually one and the same, for the prayer that Moshe offered, was actually the very idea that he intended to refer to in the name-change.
(זרע שמשון פרשתנו אות א)
