too much complexity for something simple: LCUmsuExpand=3 for boot.wim 26100+ builds works in every scenario
what is the point of adding this for W10MUI only if it's impossible to integrate 1742 LCU into boot.wim if the latest SSU was integrated first? 1742 LCU can't be integrated even into 26100.1 boot.wim the real question is if you should skip the latest SSU or not (I think the latest SSU is fine, and LCUmsuExpand=3 is fine, it just works just like msft intended)
"intended" is a exaggerated word to describe msft chaos changes with 26100 if or when they backport 26340 changes (where LCU update.cat is used to verify most sub-packages, instead .cat file for each .mum, and mumx.esd is required), then LCUmsuExpand will be dead for 26100, and "intended" msu installation will be the only choice unless we find a way to extract collection.mumx