I'd like to address this bit:

Has that pronunciation changed over the years? Does it differ from how your parents pronounced "envelope"?

Yes, and yes. When I was little I was taught "ahn-velope" and now it flip-flops. There are other words I could add to this collection - I realize now my mom retains an older form of Canadian pronunciation and my pronunciation has drifted over the years since she stopped correcting my pronunciation.

I came across an audio recording made of my family interacting in 1981 and I was fascinated with two things on that tape: (1) my pronunciation of "again" and "been" has, in many contexts, shifted to the USn version [something which I am now actively trying to change, if only to assert some small personal sovereignty over my language], and (2) my mother sounds exactly the same as she always has, both in her accent and in the fact that she spoke to us kids as if we were adults.