The end of last week's parsha sets the stage for the story of the mabul. "Va'yinachem Hashem ki asah es ha'adam ba'aretz." Netziv notes that it doesn't say "ki bara es ha'adam," like "braishis bara..." G-d did not "regret" kavyachol the creation of man. What G-d wanted to change is "ki asah es ha'adam." Asiya is a more complete stage of creation. Man needs to struggle, to work. There has to be a void for us aspire to fill. When things are done for us and we don't have enough to do, it's not good. "Ha'batalah m'vi'ah l'ydei shi'amum," Chazal tell us. Indolence breeds sloth and bad behavior.
The Kozhnitzer Magid similarly writes that if a human artisan sets out to make a vessel and it does not meet expectations, he tosses it out and starts again, but it is impossible to say the same thing about G-d. To think that G-d would destroy the world and start again because it did not work out the way he "anticipated" is an impossibility. (What about the Midrash that G-d created many worlds and destroyed them before creating ours? He does not address it. My hunch is that the difference is that only our world was created based on the blueprint of Torah, and Torah is eternal).
With this background we can understand a Rashi a little more deeply. The Midrash writes (quoted in Rashi) that the dor ha'mabul threatened that if Noach entered the ark they would to destroy it and kill him. "Va'yisgor Hashem ba'ado" means Hashem protected Noach and ensured he could enter the ark without harm. R' Shteinman in Ayeles haShachar asks what it is the dor ha'mabul were hoping to gain by this. Did they want to prevent Noach from saving himself out of sheer vindictiveness? What did Noach do to them that would warrant such a reaction? After all, he had been trying to convince them to repent and save themselves for years. Why were they out to get him?
The Kozhnitzer Magid answers that had Noach not entered the ark, the result would not have been his perishing with everyone else. Once G-d created the world, there is no undoing the act of creation -- again, G-d is not like a human artisan who tosses side a failed product to start over. There had to be a shei'ris ha'pleita of the old world; there had to be continuity. Therefore, the only possible result of Noach not entering the ark would be no one perishing. If the choice was total destruction or no destruction, the only possible outcome would have been no destruction.
Chazal (Sanhedrin 98) tell us that there are two ways Moshiach can come: he can come in a generation that proves itself completely righteous, but more amazingly, he can also come in a generation that is completely wicked. If the historical struggle between good and evil comes to the point where evil completely vanquishes good, then game over, but it does not mean the world is destroyed. It means that there is no longer any purpose to the game, and G-d will reveal himself fully (see Michtav m'Eliyahu vol1 p 28). The end of the game is the same no matter how it plays out. This was the plan of the dor ha'mabul. So long as there was a Noach, a spark of righteousness, of hope, then the status quo of schar v'onesh and the struggle between good and evil would continue. If Noach however was killed, the struggle would end in complete redemption of dor she'kulo chayav.
In his Mayan Chaim, R' Chaim Charlap (son of R' Y"M Charlap) suggests that this is what Rashi means when he writes that Noach vacillated, "ma'amin v'aino ma'amin," when it came time to enter the ark. Surely Noach, the tzadik tamim, did not harbor doubts in emunah. Yet, what Noach realized is that his very lack of doubt, his tzidkus, his belief, would destroy the world. What he realized is that he was the one thing that stood between the complete redemption of dor she'kulo chayav and a flood that would destroy most of mankind. The flood is called "mei Noach," says the Kozhnitzer Magid, because Noach's righteousness effectively doomed the world. Would it not be better under those circumstances to maybe doubt a little bit, maybe as an aveira lishma, to spare the world and bring it to redemption?
Yet paradoxically, the very thought of doing an aveira lishma to spare the world itself only enhances and proves Noach's tzidkus. Who else other than a tzadik would do an aveira for the sake of sparing a generation of such evildoers? And so G-d protected Noach as he entered the ark and the flood came.
"Va'ya'as Noach k'chol asher tzivah oso Elokim..." A perplexing diyuk: why the name Elokim, which connotes midas ha'din, when we are speaking about the means of rescue? Should the pasuk use the shem Hashem that connotes rachamim?
Based on this approach, the pasuk hits the nail right on the head. Davka because Noach had a path to rescue himself, the rest of the world was doomed and was subject to din.
(As for R' Shteimnan's second question on the Rashi, maybe you can use Sefas Emes 5641 d"h b'Rashi from "af she'lo matzinu she'pa'al bahem" to answer it.)