I thought the polite form for second pronoun was just "ön". why is it önök here ?
Ön is singular, Önök is plural. That's also why there's a plural ending on the verb (csinálnak can be either third person plural, or as it is here, polite plural.)
the hover-over translation says "they are doing" for csinalnak, is this a mistake, or are "you" and "they" the same conjugation?
The polite plural you Önök uses exactly the same verb forms as the 3rd person plural. So csinálnak could be Ők csinálnak "they are doing" or it could be Önök csinálnak "You (polite) are doing."
Thanks, you're quite patient, I'll be sure to read the grammar guide from now on...
Considering this is the same form as 3rd person plural, could this be translated as "what are they doing here ?" . It's not accepted and I'm not sure if I should report it
Mit csinálnak itt? could mean "What are you [polite, plural] doing?" or "What are they doing?"
But Önök mit csinálnak itt? could only be "What are you [polite, plural] doing?" Önök is specifically the polite, plural "you" pronoun.