dear Vidhu,
seems you have read quite a lot about the medications you are applying. although tar is carcinogenic but application of small amounts in the scalp is not particularly as hazardous. so you might safely use it without any worries. ofcourse with sunscreens since you have acne you need to apply a non comedogenic one.(which is less greasy). the parabens are the preservatives used in the sunscreen which help avaoid the degradation of the product for long term use. As long as you are not allergic to parabens they are safe to be used . there are a few general practices and life style modifications which you should incorporate like avoiding hair oil; greasy products, avoid stress, managing diet and compliance with medications. hope that helps