thanks for your question.
1. SPF 30 and above is generally sufficient, but only if you apply enough of it. It would require a small tea spoon amount of SPF 30 sunblock for the whole face. Therefore, I would recommend applying SPF 50 or more, so that you get the minimum amount of protection for lesser quantities of sunscreen applied.
2. Sunscreen lasts only 4 hours. If you will be re-exposed to strong sunlight more than 4 hours after initial sunblock application, you should ideally reapply your sunscreen.
3. You should look out for sunscreens that contain SPF for UVB protection, and UVA index for UVA protection. Both are important.
4. This really depends on the type of sunscreen you are using! Most sunscreens can be removed with water alone, but there are naturally some waterproof sunscreens as well. For those you’d require an oil-based make up remover.
Hope that helps!
Cheers
