re: Would Kurt Cobain have been more successful than Eddie Vedder?

I see Kurt this way. He was consumed by trying to attain fame. Once he got it, he didn't want it anymore. Not at the level he had it.
Pearl Jam went thru the same crap but they were way more stable than Nirvana was. They learned to live with it. Best thing that happened to them was that the casual fans only remember them for one song (Black). They continue to make music that is different from the preceding album. That's what makes PJ great, lasting, and relevant. It was a lesson learned with the overnight superstar status and it has served them well.

Do I think Nirvana could have achieved the same thing?

No. Their super hit haunted their frontman and he went thru great lengths to distance himself from it. Also, heroin is a helluva drug. Also, Courtney Love was one evil b.