Vertical stretch rephrasing

Question

From IGCSE Ch21 (quadratic) errata:

1. Is the second part supposed to be stretch of $y=-x^2$ and not $y=-ax^2$?
2. Can I say $y=ax^2$ is a vertical stretch of $y=x^2$ with invariant x-axis and scale factor $a$?
3. Can I say $y=ax^2$ is a vertical stretch of $y=\text{sgn}(a) x^2$ with invariant x-axis and scale factor $|a|$?

From IGCSE Ch21 (quadratic):

From IGCSE Ch20 (transformations):

Answer 1

1. Yes.

2. Yes, provided you have defined negative scale factor.

3. Yes.