The Netziv's introduction to Shir HaShirim deals with this a lot. People had an innate desire to connect to G-d, and for hundreds of years were able to do so in their own backyards. Then along came the Temple in Jerusalem and suddenly they were told "you can't do those anymore; you have to take all that religious passion and funnel it in a much more limited, ...
'Evil' in this context does not mean sin. It means the evil that will befall them, namely the punishments that will come as a result of their sins (see Rashi's commentary to verse 15). Tanach tries to avoid directly mentioning terrible things happening to the Jewish people, not the Jewish people sinning.
According to Sforno, yes: both pain and other issues on childbirth and pregnancy mentioned in this verse indicates that the exact opposite occurred before their sin.
her pregnancy will be exactly the opposite of what it had been
before she sinned. (Translation from Sefaria)
On regard the other question, if they had a child before they sinned, the answer is ...
The Sefer HaYovelot (or Yovelim) is the oldest existing Midrash/perush on the Chumash (that we have today), and it dates back to the times of the Hashmonaim.
It states that Adam and Hava had children only after the Chet
ובראש החודש הרביעי יצאו האדם ואשתו מגן עדן וישבו בארץ אלדד בארץ אשר נבראו שם
And on the new moon of the fourth month, Adam and his ...
Rav Avigdor Miller explained
When a person is learning gemara in the yeshiva, and it comes 1:30,
and now the mashgiach gives a knock on the table and says, “Now it’s
time for mussar,” so you close your gemara and you open a Chovos
Halevavos, and you start talking about a tree. In שער הבחינה you learn
about the trees! Will you say that he’s מתחייב ...