1)
Why does it say "Dibarti Elayich b'Shalvosayich"?
1.
Malbim: When you were in serenity, and did not yet sense your downfall, I already warned you what will be your end!
2)
What is "b'Shalvosayich"?
1.
Rashi: It is when you had serenity.
2.
Radak: It says b'Shalvosayich (plural), for they had many serenities when their deeds were good. When they began to do evil, amidst their serenity, Hashem rebuked them via Nevi'im; it did not help.
3)
Why does it say "mi'Ne'urayich"?
1.
Radak: This is Dor ha'Midbar. It was like youth for them; it was the nation's beginning to enter Avodas Hashem and His Mitzvos. The same applies to "Chesed Ne'urayich" (2:2].