I think your nurse is right. I tried to delay taking 6mp(similar drug) and my inflamation got worst. I beleive all mesalamines work fine initially but eventually they don't work as well and Aza/6mp should be next step. Your goal should be off the prednisone. If your doc tells you to try biologics, most probably he/she might want you to take Aza/6mp with it too so you don't develop antibodies. So why not try aza alone first.
It is a scary drug but lot of us take it. What you nurse did blood test is to find out you can metabolised this drug or not. If it comes fine, she might start you on it. Make sure you do complete blood count and liver function tests often.These will be your routine tests while you are on Aza.
Good Luck and wish you long remmission.