1. What can a character do in combat without a duelling school? All of the maneuvers are axclusive for members of an duellist school, arent they?

Yes. All maneuvers are exclusive to those who possess the Duelist or Student of Combat advantages.

As to what a character can do in combat without a dueling school, the answer is ANYTHING ELSE.  They can slash or stab someone with a blade (just not as effectively), crack a bottle over an opponent's head, wrap a cloak around a sword arm, pressure an opponent, throw a drink in someone's face, hit them with a chair, throw someone out a window, pretty much ANYTHING that is not defined as a maneuver.  The only thing that really distinguishes a duelist from a non duelist is wounds.  Non-duelists typically do 1 wound per raise spent, whereas a duelist will often do Weaponry in wounds per raise spent (slash), or 1 wound plus a stunt.  Don't get tied up in the somantics of the maneuvers.  Just because its listed as a maneuver does not mean a non-duelist cannot "slash at a villain with her sword", "parry a blow", or even attempt a feint (PRESSURE!).  It only means that a duelists get extra umph when they do these things.

2. A specific question about Leegstra’s Crash.
"Leegstra’s Crash deals a number of Wounds equal to your Ranks in Weaponry. A Duelist can only perform Leegstra’s Crash once per Round." ... It is basicly the same as the slash maneuver. So whats the point?

You can't perform the same maneuver more than once consecutively in a round.  You have to break them up.  So you can't SLASH/SLASH/SLASH.  However, with Leegstra, you can SLASH/CRASH/SLASH/LUNGE which is a pretty potent combination for dealing damage.  That's the point.

By 1. do you mean what non-duelists can do?  Just reread the Action Sequence section.  While its not specifically called out it's at least strongly inferred.  1 raise = 1 wound or counters 1 wound.  Everything else is just narrative dressing.