What are the implications of "Ach ba'Hashem Al Timrodu!" why they add the word "Ach"?
Seforno: If you want to acquire the land on the above terms, then don't rebel against Hashem. 1
Seforno: As the Torah writes in the second paragraph of the Sh'ma ""Vehayah Im Shano'a Tishme'u ... Venasati M'tar Artzechem ... Ve'asafta Deganecha ... ".
Why did Yehoshua and Calev make the two statements "Ach ba'Hashem Al Tumrodu!" and "ve'Atem Al Tir'u es Am ha'Aretz"?
Rashi: It is not two statements but one - 'If you will not rebel against Hashem, you will not need to fear the people of the land!'
Ramban: Having witnessed firsthand the numerous miracles of Yetzi'as Mitzrayim, displaying fear of the Cana'anim after He had promised to drive them out, was in itself, an act of rebellion.
Seforno: If you don't rebel, Hashem will bring you to the land, and you won't need not be afraid.
What did they mean when they said "Ki Lachmeinu heim"?
Rashi: They meant 'We will consume them like bread'. 1
Ramban: They meant that, not only should the people rely on the miraculous Hand of Hashem, but that, bearing in mind the fear that had gripped the Cana'anim, they were destined to fall before them by natural means. 2
Seforno: 'We will consume them like bread, and they will not (dare to) rise up against us, just as bread does not rise up against the person who is eating it. 3
And what were they referring to when they said "Sar Tzilam me'Aleihem"?
Rashi #1: They were referring to their spiritual shield and strength - Iyov, who shielded over them and who had just died. 1
Rashi #2: They meant that Hashem's protection had been removed from them.
Ramban #1 (citing Ibn Ezra) and Seforno: They meant that the Cana'anim were so terrified of Yisrael, that they could not bring themselves to pick up their shields 2 to protect themselves (and in order to run away as quickly as possible - Seforno).
Ramban #2: It refers to the shadow that appears on the head of a person on the night of Hosha'an Rabah of a person who is destined to live that year, 3 which had been removed from the Ca'ananim, a sure sign that they were doomed.
Ramban #3 (citing Midrash Shir ha'Shirim): It refers to the downfall of the angels of the Cana'ani nations, which presaged their own downfall. 4
Rashbam: 'From the time that Hashem dried-up the Yam-Suf, all the inhabitants of the land are so terrified of us 5 that they have given up all efforts to protect themselves', and are therefore left with no form of protection to rely on.
Presumably, this is because the time had arrived for Yisrael to take the land from them. Refer to Lech-L'cha, Bereishis, 15:16:1:1 & 2.
Ramban: The shade that protects every army that goes into battle.
See R. Chavel's footnotes.