I don't offer an answer to your overall question, but to answer your point 1:
We know (Rashi on Genesis 34:7) that the nations already strengthened their observance of Arayot after Noah and his daughters' accident.
it pays to look at both the local Rashi and the Rashi to which you refer. The Rashi to which you refer says:
וכן לא יעשה. לְעַנּוֹת אֶת הַבְּתוּלוֹת, שֶׁהָאֻמּוֹת גָּדְרוּ עַצְמָן מִן הָעֲרָיוֹת עַל יְדֵי הַמַּבּוּל (בראשית רבה):
That is, they refrained from rape (la'anot) of virgins, because of a general fencing off from "arayot". Thus, what Shechem did was out of bounds even by gentile standards, as a societal norm. That does not necessarily mean that unmarried young men and women did not engage in intercourse. (Is there a prohibition on a panuy with a penuya?
Then, look at the actual text of the local Rashi (on 24:16):
ואיש לא ידעה. שֶׁלֹּא כְּדַרְכָּהּ, לְפִי שֶׁבְּנוֹת הַגּוֹיִם הָיוּ מְשַׁמְּרוֹת מְקוֹם בְּתוּלֵיהֶן וּמַפְקִירוֹת עַצְמָן מִמָּקוֹם אַחֵר, הֵעִיד עַל זוֹ שֶׁנְּקִיָּה מִכֹּל:
The assumption is explicit in Rashi that the women were promiscuous in unnatural ways. Maybe you could argue that this is a dispute among midrashim, but consider that Rashi's source, in both cases, is Midrash Rabba.
Perhaps you could say that indeed, because of the increase in gentile society for sexual purity at marriage, this was why, when boys and girls were being boys and girls, the gentile girls engaged in 'unnatural acts' to preserve their maidenhead. This is what Rashi said, after all.