What did Moshe mean when he said "ve'Im Beriyah Yivra Hashem"?
Rashi: What he meant was that, if Hashem would create a new creation, they would die in a manner that nobody had ever died before. 1
Rashi #2 (citing Sanhedrim 110a) and Rashbam: He meant that, if Hashem had already created a mouth to the earth from the six days of the creation, 2 that was fine; but if not, then let Him create one now. 3
Rashi #3 (in Seifer ha'Pardes): Refer to 16:30:1:2. Bearing in mind "v'Ein Kol Chadash Tachas ha'Shamesh" (Koheles 1:9), with reference to Hashem's promise to him 4 to perform wonders for him (with direct reference to opening the earth) 5 , Moshe was requesting that Hashem put His promise into practice.
Ramban #2: It means 'If Hashem will produce a new creation. 7 Nor was it comparable to an earthquakes, where the earth remains open, and where the opening fills with pools of water, unlike the current phenomenon, where the earth actually opened its mouth, 8 and after swallowing up Korach, Dasan and Aviram, it closed it again (leaving no signs of having opened). 9
Targum Yonasan: If Hashem did not create a (unique) death for them from the day that He created the world, then let Him create one now, and if He did not create a mouth to the earth from the creation, then let Him create one now!
Rosh: If You already created Gehinom, fine; if not, create it now. This cannot truly mean to create it now, for "v'Ein Kol Chadash Tachas ha'Shamesh" (Koheles 1:9)! Rather, bring the opening close.
What are the implications of the statement "vi'Yedatem ki Ni'atzu ha'Anashim ha'Eileh Es Hashem"?
Seforno: You will see that they are not worthy of the honor that one performs with the deceased to be buried like everybody else.
Why did Moshe say that the land should swallow them?
Hadar Zekenim (5): At Matan Torah, Hashem held the mountain over them, to swallow them if they will not accept the Torah. Now that they are estranged from Torah, Hashem should create a mouth for the land to swallow them, like at Matan Torah.
Refer to 16:7:151:1*.
Why was Moshe unsure? Did he not know that Pi ha'Aretz was already created Bein ha'Shemashos (at the end of Creation)
Moshav Zekenim #1: He did not know if it was to swallow them, or others.
Moshav Zekenim #2: He thought that it was created to swallow the dead, e.g. the Mitzriyim 1 [after Keri'as Yam Suf], but not living people.