1) The EH onl is destroyed if you're DS(= not canon)
Some people said that Nihilus(?) was a Master of the exile cause they form bonds more easy.
Did you ever considered that The exile was the Master and Nihilus was the Padawan.
WE know that the exile trained The disciple for leaving with Revan for the Mandalore wars. So maybe he trained a new Padawan in the wars. Who would approve of the war. They formed a Connection and tadadididilaladida DArth Nihilus is Born on MAL V