A few of my lazy thoughts and opinions to your questions Matt;
I believe all skirts are not symmetrically perfect, hence they will never swim with out spinning/turning albeit slowly.
A hook rides sort of upwards on about 20-30 deg angle and they slowly will rock and roll sideways, albeit slowly, dependent upon speed, water movement and much more... but now with the influence of the skirts movement also. The idea of the Pakula keel weight is to make the hook rig to ride upwards as stable as possible and having a pivot point just behind the head with a ball bearing swivel. This offers that flexibility and freedom of movement to keep the lure swimming more stable with the hook up and at the ready. Versus a stiff hook rig fixed into the rear of the head and the entire system moves/spins together. Hookup rate can be greatly reduced...
Some heads are more stable than others, pushers and cup faced for instance. The heads with their glorious eyes and coloured inserts and shapes may only be appealing to fisho's because when a pusher/cup face is in the water they tend to drag/pull a big bubble around the head some of it smoking out down the lure skirt, but staying within the head until the lure pops up to the surface to breathe - so to speak - and then the cycle commences, hence not much of the head is seen by the predator. I think these lures are better in NZ & perform well in rougher conditions also, again for stability, making them presented well and easier for the predator to bite, swipe, whack and get hooked solidly.
Cut face, angled, kona heads may be better on flat calm days to create more splash, noise and excitement, hence their popularity in the tropics and the calm Ozzy waters,etc.
If you know there are fish in the area and you aren't getting bites, put out some smaller lures. Sometimes its matching the hatch and copying the baitfish in the area and on the mood of the predators on the day, if they don't want to expend their energy they may not strike at the bigger lures and will the choose smaller prey as they are easier to catch. Look at the cedar wood plugs that Atlantic fisherman troll and catch Bluefin Tuna on.
Marlin are alleged to see shades of blue, whereas Broadbill have superior eyes and can see colours, especially at depths.
Black and purple work so well because the dark silhouette contrasts very well to predators that check out the closer in white water behind the transom. So we would presume marlin come in from below and sometimes from behind, so how much detail, colour, soft white under-bellys, size of bait shape do they need to see to signal a bite? And from behind way back on the shotgun position why is an orange or pink coloured bullet shaped lure with lesser action chosen over the others, shy fish, lazy or even desperately hungry fish perhaps...?
I am quite drawn to big Nomad Bibbed Minnow lures running down below surface skirted lures to be part of the action, noise and attraction also. Teasers with hooks perhaps.
Should we run white and translucent under-belly coloured lures or something that makes a massive white smoke/bubble/air trail, combination of black and white... with fresh squid slivers wrapped under the squirts...
Lazy ideas for winter thoughts.
You decide Matt...
"The sea, once it casts its spell, holds one in its net of wonder forever." - Jacques Cousteau