Is it okay to pay visa fees before filling out the DS-160 form?
No you can’t make visa fee before submitting the DS-160, because once you submit the DS-160 form, it send a email to you with submission confirmation number which is important to synch the payments under your application. Bother how will they ever know who paid for which application.
Do I have to refill the DS-160 form? I remember having filled the "Person paying for your trip" as "Employer in the US", but when I printed out the entire DS-160, it shows a blank for that field. My biometric interview is already done.
Not important, they would know it , anyway as you have filled your application and in your interview they ask your purpose of travel. You can explain there. Just your company is sending you ans they are bearing the expense.
What should I have before filling out the DS-160 visa application form?
While filling up the US DS 160 form, keep all the documents ready as per the document checklist. Make sure to enter the same data as mentioned in your documents. There should not be any contradiction of information in the form and your documents.
How long can I take to fill and submit my DS-160 form after booking an interview appointment?
You must submit your DS-160 application online prior to making an appointment for an interview at the Consulate.Read here: Apply for a U.S. Visa
I just discovered that while filling out my DS-160 form, my passport issue and expiry dates turned out to be the same. Can this pose any issue?
You mean both the expiry and issue dates are the same? Very interesting.It means either your passport is showing as issued on a future date. Or your passport is already expired. In either case it is invalid. Best, you can get this error rectified and then proceed. Otherwise there can be problems with your visa application.