We got married in Qatar so to answer your questions,

Q1 - Yes it is mandatory
Q2 - Check with the institution where you will get married if Naseem Al Rabeeh is accredited as it is expensive to have this test done. They should have a list there. Before, the list was only Al Ahli, Al Emadi and Doha Clinic. We had ours done in Al Emadi in 2013 and it costs QR1200 per head.
Q3 - I hope you have secured both your attested singleness certificates
Q4 - Inquire with the institution where you plan to get married. They should have a list there for the steps, fees and the necessary documentations