[Aeraaa]

1) All artillery can fire directly (that I know of, at least). You simply attack a visible target like any other unit (left click, or T+cycle with N or P+F). You have to be in range obviously, some mortars may not be in range of a visible target.

I strongly advice using mortars in direct fire when engaging dug in infantry, especially 81mm and below. Not only you fire at the target in the same turn (as opposed to 1-2 turns after the current one), but mortars are much more effective in causing casualties that way, while with the indirect fire option, you will suppress the target but rarely cause any damage (heavier artillery can cause casualties even in indirect fire mode).

2) AFAIK that option means that every single unit can act as spotter (never played with it off, so maybe I'm wrong). Now, every unit has an infantry, an armor and an artillery rating. Each one of them represents how good they are at managing the respective combat arm. Regarding artillery, higher artillery values mean they call fires faster, more accurately (more chance to deal damage) and with less dispersion. So, a unit with low artillery values might take +4 turns to call a mortar strike, while a unit with high ones may take 2 turns to do the same thing. Observed fire is always more accurate than unobserved fire so if you have an observer you usually want to have observed fire. There are exceptions to the rule of course (f.e. the spotter has really low artillery values and the fire mission will take a long time to arrive). Even then, the trick is to use a good spotter to call a fast fire mission, have the round fall, then adjust with the worse observer.

3) Aircraft have to detect the target to fire at him. So when an aircraft comes, it conducts a search at the point you called it. It may hit the target you wanted, but there are cases it spots another unit and fires at it (if you are unlucky or are in close combat with the enemy, you may experience some really nasty friendly fire). Sometimes the aircraft does not spot anything, at which case it conducts a fly by the battlefield. Perfect opportunity for the enemy to shoot your aircraft down.

Also, remember to plot the route in and the route out, it may be very important in what target your aircraft sees (plus it helps in surviving AA fire).