As for whether the second attack can be debilitating:
Note that this was 23 years ago. I had been struggling for quite awhile with numbness and tingling in my legs, and weakness, particularly in my left leg and knee. I couldn't stand for any length of time: even standing for a half hour at a file cabinet, filing away papers, was too difficult. And walking? Maybe a couple of blocks, and then I'd have to sit down. So -- looking back -- clearly some signs and symptoms of MS, but I didn't know it at the time.
Then my legs started collapsing under me. I'd fall down, for no apparent reason (like, I didn't trip, wasn't knocked down...) The BIG exacerbation was this: woke up one morning and realized I could not stand up out of bed -- my legs would not support me. And I'd lost color vision in one eye, and had a very weak arm. Not to mention poor bladder control. By the time I got to the doctor and got to the hospital, I was unable to move from mid-torso to toes, lost color vision in both eyes, left eye wasn't tracking, left arm and hand were so weak I could not grasp anything, right arm was getting progressively weaker.
3 months later I was released from the hospital in a wheelchair.
Again -- this was 23 years ago. The doctor didn't know what to do to treat a serious exacerbation (these days they put you on steroids asap). There weren't any of the disease-modifying drugs (Avonex, Betaseron, etc.) that are now available. Had there been, had I started on one of them right away, I'm convinced I'd be a LOT less impaired than I am, now. And I've seen folks newly diagnosed who were experiencing early symptoms, who have not had any further serious exacerbations. So I'm a "believer" that these drugs do as they're purported to do, for many folks: slow the course and progression of the disease. A VERY good thing.