Best Of

Some programs support locking normals which makes merging two sections like that while keeping the shading much easier. Some also support copying normals from one mesh to another, which can be used similarly with just duplicating the mesh first. 

I prefer not to edit normals manually if possible.

To elaborate on that: Perfectly glossy reflections will amplify even the smallest changes in surface direction, because the angle of the reflection moves twice as much as the surface normal, and vertex interpolated normals/shading is prone to show gradients to begin with. So that's simply how it works. I agree that your top example will look fine in most use cases, and especially if you use a normal map which ideally gets rid of the vertex interpolation artifacts altogether.

If you absolutely wanted to fix the vertex shading, you could try to add some more horizontal edges to go with  the vertices of the cutout, but ultimately, you'd have to resort to editing the vertex normals. They are split around the cutout, but as the surface bends downwards, the normals interpolate downwards as well, while the ones on top of the cutout have no such influence. A practical way to get a near perfect result would be to project the outward-facing vertex normals from a shape without the cutout.

Likewise, your second examples is fine, in principle, I'd just add some more edges/vertices as needed to relieve those very tight horizontal triangles as that might require a pretty high resolution normal map for compensation otherwise.

If you're going to be baking a normal map, then using a contiguous/watertight mesh is actually preferrable. More about that here: http://wiki.polycount.com/wiki/Normal_Map_Modeling#Contiguous_Meshes

The normals don't have to look perfect on it however, because the normal map will change all that anyhow. You just want to use hard edges, and/or face-weighted normals, to reduce shading extremes which will improve the baked normal map. More about that on the same Wiki page.

You don't need to attach all your parts together in the same watertight surface. The orange knob can just be floating on top of the surface, same with the gray base+handle. This will allow you to simplify your surface considerably.

After that, you can refine your underlying surface.

However, most lowpoly assets will not look great with perfectly smooth metal materials. During modeling you can use a similar base material to what the final intent would look like, to avoid worrying over details that would never be seen in the final textured asset.