> so if lda $00,y is instructed
It would actyally be LDA ($00),Y (can't forget the parenthesis, without them, you're using zero page mode).
Anyway, in that example, A would be loaded with what's at $C005. The value of Y is added to the adress at $00 (which was $C000). If Y + the LSB is greater than $FF.. the page boundary WILL be crossed as expected (at the cost of 1 more CPU cycle).
So in your bottom example... it would be reading from $C015. But again... don't forget the parenthesis.
"LDA ($00),Y" is not the same as "LDA $00,Y"