Sorry for resurrecting old threads so much, but I'm just getting up to speed here, and putting up thoughts as I have them.
There may need to be some consideration given to zell usage in the KU. In the canon invasion, several major factors contributed to the usage of zell, even late into the Invasion.
1. The Clans were portrayed a bit more naively, holding onto that Klingon-like honor system because it benefitted the needs of the story. The Clans were also portrayed as somewhat naive when it came to non-frontline combat strategic level concerns (pre-combat intel, enemy surveillance, HUMINT, etc.) and logistical concerns. So, the heavy usage of Level 1 zell (the most restrictive), save for a few select Clans, makes a great deal of sense. This is not so much the case in the KU. While I would imagine that most Clans would easily still follow Level 2 or 3 zell (I'll play nice, so long as you do), I'm not so sure if some of the less hidebound KU Clans would even come in with that mindset. Clearly, they've had a fair amount of intel on the IS, maybe even more than in canon (between the Dragoons intel, the Terran ship captured, and the fact that the Clans own intelligence assets are not quite so lacking in the KU). That's worth considering.
2. The number of Clans involved in the Invasion and the number of Clans left alive; the differences here between canon and the KU are important. Canon Invasion had only the five Invader Clans, and of them, only 1 (Wolf) had anything resembling a non-hidebound view of zell. That isn't quite the case in the KU. First, all the Clans have survived to the modern era. That means all the Clans are accustomed to fighting other Clans that have less than traditional views on combat to varying degrees. They were not just fighting the Wolves in the homeworlds. You have the Wolverines, the Mongooses, and the Burrocks all floating around. These are all Clans clearly shown to have some very non-traditional views on combat and the prosecution of war as a logic process, rather than the artform of honor duels.
3. The Inner Sphere itself is the final factor. Outside of the units that were involved in the 4th Succession War (which granted, was a lot of them), there are many units floating around out there in the canon Invasion that were still set up to fight the last war. That would be the 3rd Succession War, where raiding and dueling (to some extent) was a necessity of keeping units active that were no longer able to be easily replaced or manufactured. Small-scale engagements that almost resembled zell duels would lead to some of the backwater units (like those the Clans first encountered in the IS houses, once they got past the Deep Periphery) still thinking in terms of the 3rd Succession War. So the honor duels may have been more easily held to (from the IS side, even unintentionally), simply due to the nature of the armies and their mindset at that time. Again, this isn't the case in the KU. You didn't have the level of destruction from the 1st and 2nd SWs (at least as far as I can tell). There's been a much more rapid return to lostech levels, SLDF era levels of production and army sizes (okay, they aren't there yet, but they are much closer than canon Invasion armies were), and the return of naval assets in a big way. So, the IS armies are now built for fighting the last war (the KU 4th Succession War), which means those armies are drastically different beasts than the canon Invasion IS armies. And the KU Clans know this.
