Try calling Yama's and tell them the cable on your Lambda is foobar. I'm guessing though that it would be cheaper to get a SRE-725 extension cable instead.
Yes, to some degree at least. The bias is the potential of the drivers so smaller drive voltages are needed but they will still be a tricky load so current is again the name of the game here.
I'm sure they were being a bit conservative so a 400v+ bias voltage could work. The bias is governed by the square of the D/S gap so if the gap is 0.4mm then 500v would work. A SR-007 works just fine at 750v...
What version of the BOM did you use? Vishay resistors or my rather nutty collection from PCX?
Those do look nice and good show for not using cones. I hate those...