I'd like to share a few thoughts. It has been almost ten years since I started studying nickel hydrogen fusion, and I believe my understanding now is far greater than even a few years ago.
Starting off, let's look at systems that use nickel rod/bar/wire and heating elements that are some distance away from the fuel and/or would not produce a significant magnetic field. My thinking is that these systems depend tremendously on achieving very high levels of hydrogen loading to produce a very brittle lattice that is primed for fracture. When the lattice is stressed by pressure or temperature changes, charge separation takes place while clusters of hydrogen migrates inwards or outwards. This produces EVOs that induce nuclear reactions and perhaps even extract some quantity of energy from the vacuum. To optimize a system like this, you should look at Focardi and Piantelli's original papers on Ni-H systems and study their surface cleaning and degassing (vacuuming) routines.
Next, we can look at early powder based systems that didn't use lithium (or at least not a huge quantity like in the Ni-LiAlH4 systems that would come later). I think that in addition to high surface area nickel particles being capable of absorbing more hydrogen due to the larger surface area - thus creating a larger embrittled surface area - the geometry of the nickel played a significant role. The spikey protrusions of carbonyl nickel would help enhance the electric fields and allow for "sparking" (EVO production) between particles. To a certain extent, these systems could produce excess heat utilizing only fracto-emission that occurs when the hydrogen embrittled lattice is damaged. To maximize this effect and boost loading to the max, if you are not supplying atomic hydrogen from an external source, you could use a nano-particle spillover catalyst like palladium or even smaller particles of nickel. But I specifically remember someone seeing a box labeled "Tesla Coil" connected to one of Rossi's early systems (right around when he revealed his tech to the world). Utilizing a Tesla coil, he could have been applying high voltages directly through the fuel - or he could have been applying sudden voltage spikes to the heating coil. Regardless, these particles would be far more likely to undergo discharges between their sharp points. Not only would transmutation or LENR take place in the gaseous environment between the tips of the particles and where the EVO eventually struck, but the strange radiation could travel to other particles in the reactor and induce LENR reactions.
Eventually, Rossi started using the Ni-LiAlH4 recipe. There's a lot to discuss here. I could mention the importance of treating the LiAlH4 in such a way to prevent it from phase changing during the release of hydrogen (to prevent the nickel particles from being smothered), the vast difference in quality between different sources of LiAlH4, the fact almost no one purifies their LiAlH4 from contaminants, or several other issues. But my first topic will be that I'm fairly confident Rossi pre-treats his nickel fuel FIRST. If I were to guess how, I'd say by running it under a corona discharge in a hydrogen or deuterium environment which would allow for the FORCEFUL application of atomic hydrogen, but this is simply a guess. Interestingly, Ni-LiAlH4 systems seem to work best - when an external magnetic field is present. And these systems seem to work even better utilizing a multi-phase rotating magnetic field at HIGHER voltages. My guess is that this magnetic field (especially rotating it with three phase) helps produce not only small discharges between the nickel particles, but also can "spin up" and sustain macro-EVOs which could be called spheromaks. Interestingly, in systems utilizing arc discharges, strange radiation usually only shows up when there is an adequate magnetic field - whether that is self generated by the discharge or by an external source. My guess is that these systems are very dependent on the type of electromagnetic stimulation that's being applied. However, if you properly hydrogenate the nickel beforehand to make sure it is already embrittled REGARDLESS if it gets smothered by the LiAlH4 or not, a less than perfect stimulation by electromagnetic field could still produce adequate results.
Then, of course, you can move on to plasma based systems. I personally don't think the QX and SK are really that complicated. I believe Rossi's extreme secrecy is due to the fact that they are extremely powerful and not overly difficult to replicate if you are willing to play around enough to produce a resonant system that sustains the complex space charge or micro-EVO. When this structure is continually existing, the signature of it, in addition to the plasma ball that's either free floating or tethered to one of the electrodes, are the ion acoustic waves that will be visible on the oscilloscope. To achieve this state, you'll need to adjust parameters such as your gas pressure, distance between electrodes (larger distance the lower the frequency, shorter distance the higher the frequency of the acoustic waves), electrical qualities of your circuit, etc.
LENR is far less mysterious to me today than it was even two years ago. All we need now are people to build the systems and test them.