Could you elaborate on the Bs thing. The only thing ive read so far is that you cannot fire a seeker missile off during overwatch but have seen no indication that one cannot fire a marker itself or use it for any other purpose.
The issue with raising the BS with Markerlights is due to the order of operations when considering modifications to characteristics. First, you multiply. Next, you make any additions. Finally, you set the characteristics to any absolute value.
Snap Shots, the sort of shooting attack one does for Overwatch, is always made at BS 1, and thus falls into the third category and, consequently, the last step. Any Markerlight boosts would have occurred in the second step, and been wiped out.
Now, there is no restriction on the negation of cover saves, the lowering of Pinning tests, and the negation of Night Fighting penalties. This means that any Markerlight hits can be used for these purposes.
HOWEVER, as stated in the Markerlight entries in our codex, ONLY networked Markerlights are able to be used by the unit that produces it. This means that, for any unit with a non-networked Markerlight (An upgraded Shas'ui Fire Warrior could be a good example), though they could fire the markerlight, their unit could NOT utilize that ML Counter in any way. They are proscribed by the codex's Markerlight entry.
Due to all Shooting being simultaneous, I'm fairly certain even a second squad (in the case of a Multiple Charge) would similarly be proscribed... but I haven't read it in enough detail to be sure.
Hope that answers the question!