The problem with Riders conclusions as possibly applicable to Polywells, is that, while he may have considered both thermalized plasma and mono energetic plasma, he did not consider the energy distribution of the electrons. Cold in the center and hot on the edge. Also, I don't think he considered a dillute concentration of boron. Perhaps ~ 10 protons for each boron. I'm uncertain how this would effect the fusion rate (at the same total density), but it would significant decrease the Bremsstrulung.
As Bremsstrulung scales as ~ 1.75power of the temperature of the electrons, and the square of the Z, the Z component of a 1:1 mixture would be ~ 1^2 + 5^2 or ~ 26. For a 10:1 mixture (at the same total density) it would be ~ 2* ( 1 ^2))+ (0.1* (5^2) = ~ 2+ 2.5 = ~ 4.5. This would be ~ 1/6th of the Bremsstrulung. This by itself would allow for a positive Q provided the fusion rate doesn't change too much.
The cold in the center electrons allow for additional gain such that some (M. Simon among them) have calculated that a positive Q of ~ 5-20 may be possible. with P-B11 fusion in a Polywell.
In a DPF the Bremsstrulung would not have the cold electrons in the center advantage. Instead, if it works it will use Bremsstrulung suppression at very high magnetic field strengths, and high efficiency X-ray energy recovery mechanisms. I understand that this might allow for final power generation gains of ~ 3-5X.
Also, I don't know if Riders X-ray losses calculation were raw numbers or if they included energy recovery through a thermal steam cycle. ~ 30% of the X-ray losses would be possible with a steam cycle. The DPF may be able to harvest much more (~80-90%?) . I don't know if a Polywell could harvest the x-rays as well.
In any case, the Bremmstrulung issue is much more benign with D-D fusion, which has some advantages over over DT fusion. And the Polywell could also gain efficiency by burning the produced tritium and/ or Helium 3. And of course D-D burning Polywells can provide the He3 for a separate D-He3 burning Polywell where aneutronic fusion(<1% fusion energy as neutrons) and perhaps some moderate efficiency direct conversion is desired.
I don't know if a DPF could survive long enough with D-D fusion because of the high neutron flux in these small machines.
Most of the above (except the cold core electrons?) may apply to the FRC also.
My limited understanding of Tokamaks is that positive Q fusion is probably obtainable. The problems may be the diverter and tritium production requirements. And, the size and associated costs may end up being the biggest road block to commercial Tokamak development.
To error is human... and I'm very human.