3. Are they suitable for use behind the cooktop in a kitchen?

Generally Yes. Tic Tac Tiles are humidity & heat resistant. They will not be damaged by hot oil. So they are suitable for use in kitchen or bathroom splashbacks.
One precaution – To install Tic Tac Tiles behind a Gas Cooktop without a protective rear panel, leave an area of 20 cm between the tiles and any open flame.