as you havent touched anything electrical,just caused by jump starting,as the initial problem,I would look at the alarm especially the earth side,try disconnecting the alarm unit connector,it sounds as if it is shorted to earth,providing an earth to other car functions,horn,hazards,also these functions are usually live without ignition on,hence original jump start problem.
you could also pick one circuit ie horn,disconnect the horn earth (the line to the horn switch),replace the fuse ,if horn still on then earth problem is pre relay ,if no horn then earth is in that line,trace that wire back to problem which i suspect will go to the alarm. does that make any sense?