3. What freq are you running at?
275/0815 target, last night's test I ran five that way, then the one I dropped to 268/0815.
Did you try 275/0800 before 275/0815?
The datasheet states 2 voltages for that freq, 0800 and 0850. You should try 0800 first and increment if you get LOTS of HW errors or a very low hash-rate. For freq 275, you should have hit at least 550 Gh/s(avg) in the hour IF your HW % is low and no x's showing.
EDIT: Saying that, the 538 you are getting with no HW errors is not bad either but I should expect your rig to give at least 550.
PS. I think you'll have to start one machine at a time .... also, I'll address the other bits as and when ...