Smallville series finale?

I just finished watching the series finale and I was wondering how Jimmy Olsen found his way back into the plot after dying several seasons before. How did he come back to life? Did the writers just choose to resurrect him? Also, I find it odd that Lois and Clark waited seven years to get married. I am also confused on the scene where Tess erased Lex's memory. Was that necessary? And how did she do it? But the finale was great and I'm sad to see the show end.