The PLL at the Y end has been locked for about 30 minutes.  I've changed the slow servo back to the old code, which has a simple 1/f response if Pf is set to zero.  
The settings are:  
In 1 Gain = -15 dB 
fast gain 4dB
Slow servo "ugf" -0.055
Pf=0

With these settings the UGF is 14.3kHz, the phase at the ugf is -100 deg.  From the transfer function it looks as though higher gains would be possible, but if you add more than 3 dB of gain an oscillation starts.  As far as I know there are no diagnostics in the slow control system to detect the oscillation.