If Hitler had been assassinated, would it have changed much?
It wouldn`t. Stauffenberg and his mates intended to make peace with Western Allies and continue the war against the Soviet Union. But it was wishful thinking - allies were determined to keep their unity till Germany was defeated.
Why Hitler built it and why he spend there so many time during war?
Those bunkers, although blown up by Germans before their retreat from Eastern Prussia, still look like impressive structures. Regular bombing of the area would have been useless, so Hitler felt safe there. The only way to get him there was insider job, indeed.