Yeah, this kind of thing is tough, and requires a lot of trial and error. If you can, I think it would help to make the radii of the 3 sided hole larger. To do that you'd probably have to make a sketch on the top plane and control that shape a little more intentionally. The problems seem to have a lot to do with how much the tangent direction changes around the open loop. It might do better if you could get something on the original feature that starts to go in the direction of the feature that you want to create. I would also use a boundary surface instead of a loft, and remove the guide curve, and use maybe one or two more closed loop splines in direction 1.
I tried a few simple things, and didn't really improve it much. I think in the end, you're gonna have to tinker with the shape and size of that trimmed opening.
best of luck.
First off, the way that you've constructed your spline (in sketch18) is not tangent with the adjacent face (see image below.) Same goes for sketch19. Use the intersection curve sketch feature to create your reference to make tangent or C2 to. then past that, use boundary instead of loft. Also creating some additional 2nd direction curves will help. See attached.
This is one of the first steps in my journey into the world of splines...
The intersection curve looks like a great tool and I didn't know about the equal curvature relation until today (because I have just started using splines).