So it would therefore not be suppressed if it was firing at a target it did not have line of sight to, i.e. b)? Because markerlights don't give line of sight, they only allow it to fire when it normally wouldn't? And logically also indirect fire weapons cannot be suppressed when firing indirectly? Sounds dodgy
Don't think it is that simple really. Remember also that the markerlights/GM rule doesn't only allow them to fire when they normally wouldn't, it also prevents them from firing when they normally would.
The fact is indirect fire overrides the line of sight rule somehow, and I have never seen it played that this does not also apply to suppression. It's obvious those units should be suppressed. The difference with tau is that this situation arises in more unusual circumstances because it is not only active during the formation's own specific (sustain) action, and also uniquely, GMs can work in the opposite direction. I can't really see a way to do it so that there is a consistent application of the rules.
Maybe some scenarios help:
6 fire warriors, 3 devilfish and a skyray sustain at some LVs without markerlights. The tanks are at the back, outside of 30, with 4 BMs. Everything has line of sight except the skyray. How many fire warriors can fire?
A) 6 - the tanks might not be able to fire but they don't ever need line of sight so count for suppression whether markerlit or not
B) 3 - the GMs only count as having line of sight if the target is markerlit
C) 5 - the GMs need line of sight to be suppressed, and the skyray doesn't qualify even though they don't need line of sight to fire.
Now imagine it is overwatch instead.
Now imagine the target is markerlit by another formation. Does this change anything?