Does an approved ETA-IL guarantee I will be let into Israel?

No. An ETA-IL authorizes you to travel to Israel and board your flight, but it does not guarantee entry. Israeli border officers conduct individual interviews with all arriving travelers and retain full discretion to refuse entry. An approved ETA-IL is a necessary but not sufficient condition for entering Israel. Make sure you can clearly explain your purpose of visit, where you will stay, and your intended length of stay.