HQMS-K salt is a very slow acting mild version of HQ itself. It is also very very stable. In this case, where you want all of the color layers to start to develop at about the same time and continue to do so, HQMS is virtually ideal when compared to HQ itself. This discovery led to a change in the E6 process very early on, to prevent overdevelopment of the yellow layer and/or underdevelopment of the cyan layer. In current products, balanced for HQMS the top and bottom layers of all E6 products from Kodak and Fuji expect this balancing act to be present in the first developer.
I might add that HQMS, to achieve this balancing act, is present in very high concentration. I think it is between 25 and 50 g/l.
ChloroHQ is very active and not in general use anywhere due to the extreme expense. I would have to look it up in my notes to be sure.