How was it possible for the entire congregation to stand in front of the rock?
Rashi: This is one of the places where a small space contained many people. 1
See Ba'al ha'Turim.
Why did Moshe and Aharon call the people "ha'Morim"?
Rashi and Rashbam: The word means both 'rebels' (or people who refuse to listen, or fools) and 'teachers', who instruct their teachers. 1 (He pointed to Aharon's staff with its almonds - symbolizing "B'nei Meri" 2 - rebels - Rashbam).
Ramban (citing Rambam in Moreh Nevuchim): It means 'rebels', and this was Moshe's sin - displaying anger. 3
Why did Moshe insert the word "Shim'u Na h'Morim"?
Targum Onkelos and Targum Yonasan: "Na" here means (not 'please' but) 'now'. 1
As it often does. See for example, Targum Onkelos on Bamidbar 12:13.
What did Moshe mean when he said "ha'Min ha'Sela ha'Zeh Notzi lachem Mayim"?
Rashi: When the well ceased to function, the rock that contained it went and placed itself among the other rocks in the vicinity; and when Moshe and Aharon failed to recognize it, the people impatiently asked him what difference it would make from which rock they would produce water, to which they replied 'How can we produce water from a rock that Hashem did not command us'? 1
Ramban #1 (citing the Radak): It is a rhetorical question, 2 implying that they would indeed produce water from the rock.
Ramban #2 (citing the Ibn Ezra): They meant to ask 3 how they (Moshe and Aharon) were able to produce water, implying that only Hashem was capable of doing that 4 - and that He was the One who took them out of Egypt, He was the One who brought them to 'this bad place' and who would sustain them there.
Ramban #3: He was asking them what they thought about Hakadosh-Baruch-Hu; whether He would produce water from the rock or not. 5
Ramban #4 (in Pasuk 8, citing R. Chananel): This was Moshe and Aharon's sin, in that they said 'Notzi' instead of 'Yotzi', (inadvertently) attributing the miracle to themselves 6 rather than to Hashem. 7 And not seeing a sign that Hashem was there, 8 some people believed that it was Moshe and Aharon who produced water from the rock.
Rashbam: As Moshe raised his hand and struck the rock twice, he said angrily - in doubt - ?Do you really think that Hashem will produce water for you from this rock?? 9
Michtav me'Eliyahu (2, p.253, citing Aderes Eliyahu): Moshe had a Safek about what should be learned from speaking to the rock, therefore he had a Safek about which rock (the rock hid itself among other rocks). 10
See Ramban, who maintains that it is inconceivable for Moshe, the master of all prophets, to err in a prophecy.
Ramban: As we find sometimes in Tanach; See for example, Sh'muel 1, 2:27 & Sh'muel 2, 15:27.
Ramban: In reply to the people blaming him for bringing them to such a bad place. Refer to 20:3:1:1.
And that He was about to do so now!
Ramban: The 'or not' is self-understood, because he was asking whether they didn't believe that Hashem could do it, as we find in other places - See for example, Bereishis 24:5 and Divrei Hayamim 2, 18:5. In fact, after calling them rebels, he was now accusing them of being Ketanei Emunah - because it was obvious that Hashem could produce water from the rock - just as David ha'Melech said in Tehilim, 78:18-20 in connection with this very same incident.
Ramban: Which explains why the Torah writes in Devarim 32:51 "Ya'an Lo Kidashtem Osi" and "Me'altem Bi" (See Ramban, DH 've'ha'Karov min ha'Devarim').
Ramban: As they did on all other occasions - See for example, Sh'mos 16:8.
Ramban: As they did in Refidim (See Sh'mos 17:6).
Refer to 20:10:4:1*.
Michtav me'Eliyahu: Since they lost this Limud, they lost also others. "V'Lo Yada Ish Es Kevuraso" (Devarim 34:6) ? they lost what could have been learned from knowing his burial site.