I love my parents!
Here's what my dad said when I expressed worries at putting my kids in daycare:
"the baby thinks it's fine--she has a good time wherever she is. It's Mom who suffers and struggles no matter what her decision."
I think you handled it fine, but I also think you should feel free to just hang up on her if she does anything close again. Well, do *say* something, like "That's a particularly nasty thing to say, Mom. I'm getting off the phone now." (I think if you announce you're leaving the call, it's not technically 'hanging up on someone.')
I wouldn't even get into "we are the parents" or "we'll be making decisions our way." I'd just say, "Mom, that's out of line. I'm getting off the phone now."
Also remember--when someone goes that far off the rails (it did surprise you), it's probably about themselves and some anxiety they're facing (or have created in their own head). It's not about any rational, sensible assessment of the situation.
But I think you still want to cut her off--she's a grownup, she should learn to handle her own anxieties without spreading them all over YOUR world.