How do I keep a lofted twist from going lumpy near the top?

I'd push back on the tolerance theory. Wavy surfaces two thirds up a twisted loft are almost never a tolerance problem, they're a sketch structure problem. If your lower and upper profiles don't have the same number of segments and matching vertices, SolidWorks has to guess where things line up and that's where the waviness comes from. Count your splines and line pieces in each profile and make sure they pair up. I fought a similar twist on a handrail job last year and the fix was rebuilding one profile with the same segment count, not hunting a settings answer. As for the open top, I'd skip the single sketch with both profiles and just loft the outside, then shell it. Two clean features beat one clever one.

Same experiance here with matching segments. I did a trophy-style part last year and the waviness went away once I added split points to both profiles so each spline had a matching partner, and I dragged the little connectors to line them up instead of trusting the defaults. Also worth checking that your guide curves actualy touch each profile, a tiny gap makes SolidWorks improvise. On the hollow top, I traced inner and outer loops in the same sketches and it worked fine, no shell needed.

One thing that worked for me on a similar twisted cup project was splitting the loft in half with an intermediate sketch at the midpoint. Two shorter lofts stacked on the same profile gave the surface a reset point, and the waviness never had a chance to build up the way it does over one long twist. I know it feels like giving up on a single clean feature, but the geometry reads the same once it is all knitted togehter. As for the open top, I would model only the outer skin and then use thicken to give it a wall, because shell on a curved loft body has failed on me more times than I can count.

For the hollow part, I had luck tracing the inner and outer edge as two closed loops in the same profile sketch, top and bottom. When both loops are there and connect properly, the loft builds the wall thickness itself instead of capping it. My first attempt still capped because I'd only picked the outer loop in the top profile and never noticed. Once I went back and confirmed both loops were highlighted in every profile, the opening went clean all the way thru. Zoom way in on each sketch before you hit the green check, saved me from another hour of staring.