This is a law of Qatar, who come for visit visa they should have return ticket up to their home country. Example: If any problem occurs while your family entered in Qatar related document wise, Medical test..or Money Issue etc... then authorities to send your family using that ticket. Hope you understand. Changing visa from visit to Resident will be different than ticket issue.

And, Flat agreement is required based on immigiration offices, better to apply Al Khor or Al wakra offices, may you get without flat agreement.