1. Is a Sump essential for a 20g FOWLR set-up?
a) Yes and No, as a "Sump" is mainly an area where you could house your equipments and make your maintance alot more easier.
Having a "Sump" do help too, when you are dosing additives. As it is better to dose additives into your "Sump" rather than directly into your tank.
2. Do I still need a protein skimmer? What kind of protein skimmer?
b) It would be better to have at least a protein skimmer. But since it is only 20gallon FOWLR. You could just control your feeding and do weekly water change with salt mix.
3. I have no choice but to use water coming from NAWASA. What would I do?
c) You could purchase salt mix from many brands like "Coralife" etc.
4. I have an OHF already is it sufficient for a 20g FOWLR set-up?
d) An OHF is more like a mechnical filter, where you could include wools, crabon etc. My 2cents, i would just do without the OHF and let the tank mature by itself.
5. What kind of lightning fixtures could you suggest?
e) For FOWLR you could go with either PL / PC (power compact) or T8.
Hope it helps. Above are just my 2 cents.
Sherlyne