No harmful effects. Sunscreen should be applied before your initial exposure to the sun, then should be applied several times during the day, especially if you are perspiring a lot. The only negative side effect is theoretical vitamin d deficiency if you have no exposure at all to ultraviolet light, so a little sun exposure may be desirable. Moderation is the best policy.
Perhaps. The biggest harm is that you think you can still bake in the sun. Sun protectors only decrease the amount of radiation exposure. You still are damaging the skin and underlying tissues.
Low vitamin D level. Wearing a lot of sunscreen is unlikely to cause any problems unless a person is allergic to or sensitive to some chemical in the sunscreen. Without sunlight hitting the skin directly, a person will have less vitamin d. If he is not drinking milk nor taking vitamin d supplements, he may develop vitamin d deficiency. A doctor can check the person's blood vitamin d level.