I haven't yet found a good analysis of the spurious output of a SDM. I spent lot's of time doing SDM simulaiton's in Simulink, and as long as I had the SDM topology correct, the output of the sdm was spur free as long as I stayed away from rational division ration (1/2, 1/4, etc). You are doing this in ADS? Are you using the SDM in the Fractional-N example they provide?
From a PLL point of view, spurious components at the output of the SDM will show up as a noise source that is within the loop filter BW. In other words, the spurious output will be low pass filtered by the Loop Response. Inside the loop response, it should directly modulate the VCO output.
As far as dead zone in a PFD in a Fractional N Loop, my opinion is that it really doesn't matter. Because the division ratio is continiously changing, the PLL never truly acheive "Lock" where there is zero phase difference between the two inputs to the PFD. With that the case, I don't think that a dead zone will be significant.
Dave