Newbie Questions

[Mowgli]

1) Perhaps the unit that is spotted that is outside of the LOS of your units LOS has been spotted by another one of your units that does have a LOS on that hex, without being able to see the units on a map it is difficult to comment, but someone must be able to see that hex, enemy units spotted by dust spotting are shown with a ? on their counter. 
2) No, LOS is updated at the start of your next move so on your present turn you may bump into hidden units or units appear if they fire ranged weapons from more than one hex away, so using Recon spotting you can attempt to reveal hidden units on your present turn before you bump into them.
3) Good question, understanding assault results can be a bit of a grey area and reading the rules does not really help, the casualties are a result of a whole host of factors including a high and low "dice roll" so you can see quite large variations in casualties and fatigue, but as a rule of thumb the winning side usually (but not always) suffers from fewer casualties.
4) I think the general consensus is that the casualties caused are the same both ways, however if you fire six times rather than three you have more chances of causing a disruption result, however this approach also has the downside of allowing more defensive fire shots back from the defending units.
5) You will find that 95% of the guys here play via the default turn system where all defensive fire is handled by the computer, this system speeds up the game considerably. Multiple phases may work in very small scenarios but would be impossibly unwieldy in the larger ones.

Thank you (and all the others!) for the answers! :) 

@ 2 (and also 1): That's good to know. The manual doesn't mention this anywhere. So spotting only happens at the start of the turn (or - in manual defensive fire mode - at the start of the movement phase?). Also, enemy units within theoretical line of sight who open fire (opportunity / defensive fire) are immediately spotted. In addition, you can use "recon spotting" during your turn (obviously you first move the recon unit, then activate the recon spotting)

@3: I've also taken a look at the Squad Battles series. And here, it is quite clear that assault casualties (or, more precise: casualty points which are later translated into casualties) are distributed amongst all the units who participate in an assault, weighted according to their strength (the manual is quite detailed on how to determine the "weight" of vehicles). So I suppose a similar mechanism is used by the Panzer Campaigns series. 

@ 4: Very good point about the increased oppportunities for enemy opportunity fire (if you don't play in phases, that is). As you say, I was just wondering if the game adds the fire power of several units to perform a single, "stronger" fire action (increased chances to cause casualties and force a morale check) or reports the sum of the casualties caused by several fire actions of the individual units (several "weaker" fire actions). It's not an unimportant question. For example, the way I understand the manual, the larger (1/2/3 component units) a target unit is, the more casualties need to be inflicted within/by a SINGLE fire action in order to force a morale check on it. So in this case, you want to pack maximum fire power into a single fire action. (For the same and other reasons, using units either in a combined or split-up manner seems to be a rather important decision...)

@ 5: I see. But what about the intermediate method "manual defensive fire" mode with optional "automatic defensive fire" turned on? 

Also thanks for all the recommendations! I've started with Sicily, as that's the campaign I know most about (if only I could modify the Primosole map!). I will take a closer look at Smolensk and Budapest. What about the France 40 title? Early war appeals to me. Are the scenarios comparatively large?