When Sessions recused Rod Rosenstein became acting AG for all matters that Sessions was barred from due to his recusal. Rosenstein hired Mueller. Without Sessions recusal he would not have had the authority to do so.
Try reading foldspace’s graphic about Sessions.
Certain things had to happen. The enemy was determined to bring Trump down via the Russia hoax so it would have happened no matter what.
Blaming Sessions is just the wrong tack. Q+ thanks to Sessions says it all.