Q1: Does the handle lock up or is it loose?
A: The handle is very cleverly designed that you can confidently leave it in the upward position if you wish as it has a lip that the handle can rotate into. This lip has been specifically made to accommodate a upward position. Otherwise you can rotate to where you like.
Q2: The stainless steel description is double. Please answer accurately whether it is 304 or 18-8.
A: SUS304 is equal to 18-8 stainless steel. So it's 304 and yes, it's also 18-8.
Q3: Does this kettle whistle when boiling?
A: No, there is no small whistling piece in the pour spout.
Q4: Do you know if this kettle is compatible with an induction cooktop?
A: If you don't mind that the kettle's bottom isn't fully flat, you may use it on an induction burner.