Well, I think that your idea of the units being formed by the Von Rohrs and then disbanded would work, except that I don't see why Takiro would have brought them back, and made them the preemininet DCMS 'mech regiments if they'd been tainted by the Von Rohrs stink. Just doesn't feel like the sort of thing which would happen in the tradition-bound DC, though if anyone would have done it, it'd be the forward-thinking (and extremely long-lived) Takiro.
Again, I think 1698 is self-contradictory enough that it's possible the SoL were "just another 'mech unit" until Takiro wanted to make them the "Soul of the DCMS"; no history needs to be rewritten, just clarified. It would allow the SoLs to still have been around for both the battle on Nox in 2475 and the RWR campaign of the Reunification War, but explain why, for instance, the two SoLs sent to the RWR were only sent to the Rim Worlds, and not the higher-priority (for the DCMS) fight in the Outworlds Alliance -- at the time, they were just a typical free-floating regiment, no different from, say, the Prosperina Hussars or the Arkab Legions.
All this would take would be reinterpreting the first line on 1698's page 82 to mean "The Sword of Light regiments were formed into their current role and status during General Aleksandr Kerensky's regency over the Star League, when increasing doubts about the League's stability led the Great Houses to engage in a general military buildup." Otherwise, we still have the contradiction from the next paragraph where it talks about the "existing Sword of Light regiments".