I'm guessing you'll be fine. Your doing your homework and sound no different than I many moons ago. I wasnt and am not a climber, but I love cross country backpacking and haved fared pretty well by just being cautious. IMO, the biggest thing as others have said is to be flexible and have alternatives. If it doesnt look safe, turn around.
I speak from experience on the Amphitheater route you are talking of. I did that hike back in 1980 (give or take a year ) with my brother and friend Tom (CI_Seawolf), but we did not cross Cataract and we had planned to. The pics by Mike & Maverick bring back memories as I have gazed upon both views, but the view from below looked mighty different than in Mikes pics. As JTCarter pointed out, there can be a snow cornice and when we did it, it was freaking massive (no less than 25 feet and likely more). When we got there we basically figured it was impassible. My brother and I dropped our packs and hiked up to it, and found we could access the pass as there was space between the ice and the rock wall that we could get through to access the summit. I as surprised when off in the distance we could see the orange of someones tent between the two western most lakes. As for crossing the pass, we knew it would be quite the trek and would be hard on all of us. We considered it, but Tom basically said "your outta your mind if you think I'm going up that." He was right, it would have been a difficult push with packs for all of us, so we stayed that night and enjoyed the solitude of Amphitheater Lake. The next day we backtracked back to the JMT and hiked to Upper Palisade Lake, the next day to Marion, the next to Twin, and then out over Sawmill (we are primarily fisherman so all of our destinations involved ending by a lake or stram) It was an awesome trip and one I will always remember. The changing the course didnt dampen the mood or the trip one bit.
All my life I've been waiting for
I've been praying for
For the people to say
That we don't wanna fight no more
There will be no more wars
And our children will play